The intento of this large commit is to move away from unittest
into pytest.

To speed-up migration we made certain choices that might
need to be revisted in the future:
- use pytest-subtests to replace unittest subtest infrastructure
  A lot of this might be better using parametrize
- While we have removed certain classes and flatten them to
  functions, some were kept for no particular reason
  (rewriting the QuantityTestCase base class)
- We have created some fixtures but not optimized.
  Feature optimization (i.e. using global registries for certain
  tests might speed up testing as it will void constructing
  the registry over and over)