- Set build-backend so isolated pip install works
- Add pychop as testing dependency
- Configure tox-conda
- Create __init__.py files under tests/ so that all tests are
  found by pytest

We need to use conda to install Mantid and check results against Abins
functions. Tox-conda is stuck on tox v3; there is a branch to support
v4 but it's not clear if/when this will be finished. But we also might
be able to drop the conda component of these tests once we are
satisfied with validation against Mantid: it can be replaced with
tests against reference values.

Perhaps we don't need the test dependencies at all in pyproject.toml,
now this is all defined in tox.ini. But they could be useful for
running tests without tox if desired.

- Use reservoir sampling to efficiently grab a subset of Fermi chopper
  frequency combinations
- Simplify error matching with match= argument
- Get id on the fly with a function rather than precomputing

- defer string ids to a function run by test
- replace explicit list-extending with itertools.chain

The sweep of incident energies is cut down by some more (seeded)
random sampling.

At the moment we have some cases with invalid chopper frequencies;
make sure the error is checked robustly when handling these.

Ideally we should have smarter test-case generation and check the
errors in a separate test

These aren't currently being checked in CI because tox takes care of
it, so can easily become stale. There must be a better way.

We are seeing some interesting inconsistencies in test results between
Rastislav's local Windows installation and my Linux/tox setup.

We know some tests are failing regardless, but want to see what
happens on Windows!

When testing on Linux, Abins returns NaN rather than raise LinAlgError
when the chopper settings are out of range. Our test should detect
both cases and ensure appropriate behaviour in corresponding
situation. (i.e. raise NoTransmissionError)

This is a bit cleaner than having separate checks for different
OS/Library situations

There is a unit-conversion bug in the low-frequency limit so this
doesn't kick in at the right place in Abins. When that is fixed we can
de-skip the test and verify it here.