Introduce ContextDimension

[LecrisUT]

This is quite a big one, but I will keep the commits relatively logically contained for easier review commit by commit. The idea here is:

• Add a container for defining the context dimension comparison operators: ContextDimension
• Users can extend ContextDimension by sub-classing it and defining the _dimension_name it is linked to
• If a dimension is not found in the sub-classes, the create_default is used, which by default wraps around ContextValue for backwards-compatibility
• All of the comparator logic was moved to ContextDimension instead. Operators and decorators were added to manage these more dynamically
• Some cleanups
  • staticmethod -> classmethod to allow expanding the Context class
  • re_expression_triple was made a function for the reason above
  • Dropped some dead-code parts

Closes #289
Depends-on #297

---

Checklist

• implement the feature
• write the documentation
• extend the test coverage
• mention the version
• include a release note

[tcornell-bus]

blocked by #294

[thrix]

This does not look good:

❯ python3 -c 'import fmf; fmf.Context("module = perl:5.28").matches("module == perl")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import fmf; fmf.Context("module = perl:5.28").matches("module == perl")
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/var/home/thrix/git/github.com/teemtee/fmf/fmf/context.py", line 752, in matches
    result = self.evaluate(expression)
  File "/var/home/thrix/git/github.com/teemtee/fmf/fmf/context.py", line 805, in evaluate
    return self._op_core(dimension_name, values, operator)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/home/thrix/git/github.com/teemtee/fmf/fmf/context.py", line 541, in _op_core
    assert isinstance(dimension_value, ContextDimension)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

[happz]

LGTM, in general the operator-based approach fits my view how these things should be implemented. I guess I will have a comment or two, mostly minor ones, when I get back in the evening.

[thrix]

Nothing else found. LGTM

[psss]

Looks good. Just the missing release note.