Add neutron inelastic scattering law unit tests

[massimolarsen]

Adds basic unit tests for the neutron inelastic scattering laws from the MCNP 6.3.1 manual (https://mcnp.lanl.gov/manual.html) in transport/distribution.py.

Each test uses a set of test data with known distributions and deterministic rng sequences to compute expected analytic values of E and mu. The expected values are then compared with the sampled values from MCDC.

All the variable names and equations are pulled from the MCNP manual to hopefully keep it readable.

Currently includes:

• Law 3: level scattering
• Law 4: tabulated distribution
• Law 4: multi-table distribution
• Law 7: simple Maxwell fission spectrum
• Law 9: evaporation spectrum
• Law 44: Kalbach-87 correlated energy-angle scattering
• Law 61: tabulated correlated energy-angle scattering
• Law 66ish: reduced N-body sampling (just checks the current implementation)

Verify with:

pytest test/unit/transport/distributions