Define radial-collision physics acceptance tests

[krystophny]

PR #205 changes collision physics through radial plasma profiles. It needs independent physics checks before merge.

Acceptance:

• Flat radial profiles reproduce the old collision coefficients within stated tolerance.
• Maxwellian fixed-point behavior is tested.
• Pitch and energy moment relaxation are compared with an analytic or independent reference.
• Seeded stochastic tests are reproducible.
• The PR states how this relates to issue Adopt libneo species and collision_freqs instead of own implementations #335 and libneo collision/species ownership.