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

noxfile.py

@@ -231,6 +231,7 @@ TEST_DEPENDENCIES = (
     "pytest",
     "pytest-cov",
     "pytest-randomly",
+    "pytest-repeat",
     "semver",
     'typing-extensions; python_version < "3.11"',  # to support Python 3.9 & 3.10
     "xdoctest",
@@ -290,6 +291,7 @@ def test_coverage_run(session: nox.Session) -> None:
     session.install(".")
     install_pinned(session, "coverage", *TEST_DEPENDENCIES)
 
+    session.env["N_RANDOM_DRAWS"] = "10"
     session.env["NO_CROSS_REFERENCE"] = "true"
     session.run(
         "python",