Under no circunstances a registry should be instantiated in a
module outside a fixture to avoid error during collection.
This precludes running simple tests that do not depend on
the registry.

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)

Each test class captures the logging output for warnings and check at tear down that
there was not a warning. If any warning is expected you must explicitly capture within
the test using `with self.capture_log() as buffer:`

Also created a contextmanager to capture log outputs.

The nullspace of the dimensionality matrix is now obtained without NumPy assistance.
This eliminates NumPy as a requirement to use the Pi Theorem. Moreover, the implemented
gaussian elimination uses fractions (not floats) by default which is more proper for
the type of matrices usually found in this application.