Resolves TopoToolbox#151

This PR adds two functions to the IOtools:

- `ttcachedir` returns the name of a platform-specific cache directory
- `ttclearcache` deletes that directory to clear the cache

`readopentopo`, `readexample` and `readopenalti` are modified to use
`ttcachedir` instead of `tempname` when generating the files.

The new behavior of `ttcachedir`, `readopentopo` and `readexample`
follows that of their counterparts in pytopotoolbox. The default cache
directories are

- Windows: %LOCALAPPDATA%\topotoolbox
- macOS: $HOME/Library/Caches/topotoolbox
- Linux: $XDG_CACHE_HOME/topotoolbox (defaults to
         $HOME/.cache/topotoolbox if $XDG_CACHE_HOME is undefined)

If the necessary environment variables (i.e. %LOCALAPPDATA%) don't
exist, an error is thrown, but if the directory doesn't exist, it is
created by `ttcachedir`. I've included an option for the user to
supply a custom cache directory to `ttcachedir` and `ttclearcache`,
but this option is not exposed at the moment to `readopentopo`,
etc. because I didn't want to modify the input arguments to those
functions too much. If the cache directory doesn't exist when you call
`readopentopo`, you'll get an error from `ttcachedir`, which is
probably more confusing than it should be.

The default name of the OpenTopography files is changed to match
pytopotoolbox, which in turn follows the CSDMS bmi_topography tool's
naming convention. It is

OpenTopo_$SOUTH_$NORTH_$WEST_$EAST_$DEMTYPE.tif

This could use more testing, especially on Windows and macOS
platforms.

Signed-off-by: William Kearney <[email protected]>