Add `Q`, `R`, `C`, and `GF2` fields

- add `lalib.fields.base.Field`, a blueprint for all concrete fields,
  providing a unified interface to be used outside of the
  `lalib.fields` sub-package
- implement `lalib.fields.complex_.ComplexField`, or `C` for short,
  the field over the complex numbers (modeled as `complex` numbers)
- implement `lalib.fields.galois.GaloisField2`, or `GF2` for short,
  the (finite) field over the two elements `one` and `zero`
  + adapt `lalib.elements.galois.GF2Element.__eq__()` to return
    `NotImplemented` instead of `False` for non-castable `other`s
    => this fixes a minor issue with `pytest.approx()`
- implement `lalib.fields.rational.RationalField`, or `Q` for short,
  the field over the rational numbers (modeled as `fractions.Fraction`s)
- implement `lalib.fields.real.RealField`, or `R` for short,
  the field over the real numbers (modeled as `float`s)
- organize top-level imports for `lalib.fields`,
  making `Q`, `R`, `C`, and `GF2` importable with
  `from lalib.fields import *`
- provide extensive unit and integration tests for the new objects:
  + test generic and common behavior in `tests.fields.test_base`
  + test specific behavior is other modules
  + test the well-known math axioms for all fields (integration tests)
  + test the new objects' docstrings
  + add "pytest-repeat" to run randomized tests many times