…soft#412)

## Fixes microsoft#111

> The modulo operator on this calculator gives the result that is different to the most used calculators.

The current `modrate` function is the equivalent of rem(...)/remainder(...), not mod(...)/modulo(...) available in some popular Math apps. 

### Description of the changes:
- rename `modrate` in `remrate` to be more accurate.
- add `modrate`, calculating modulo similarly to Matlab, Bing, Google calculator, Maxima, Wolfram Alpha and Microsoft Excel 
- Add `RationalMath::Mod` using `modrate` as an alternative to `Rational::operator%` using `remrate`
- Add a helper `SIGN` to retrieve the sign of a `Rational`.
- modify `CalcEngine` to use `modrate` in Normal and Scientific mode and `remrate` in Programmer mode.

### How changes were validated:
- manually and unit tests added

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.

These are the changes extracted from microsoft#211 that port number's mantissa
from flexible member arrays, a non-standard C++ extension, to
std::vector, which enables compilation and linkage with clang.

@fwcd is the original author of these changes, I (@janisozaur) merely
rebased them on top of current master.

The rebase was luckily fairly straight-forward, with the only real
conflict coming from 7a7ceb5
(microsoft#412), but was trivial to
fix.