Adopt libneo species and collision_freqs instead of own implementations

[krystophny]

Context

The libneo dependency audit (itpplasma/libneo#258) identified that GORILLA has its own implementations of species and collision frequency calculations. The team decided that downstream codes should adopt the shared, tested implementations from libneo to reduce code duplication.

Note: GORILLA currently does not depend on libneo for the collision chain, but uses libneo for other functionality. Adopting the shared physics modules would reduce maintenance burden and ensure consistency.

Action

1. Identify existing species and collision frequency code within GORILLA
2. Replace with libneo's shared implementations (LIBNEO::species, LIBNEO::collision_freqs) via FetchContent
3. Remove duplicate code

Ref: itpplasma/libneo#258, itpplasma/libneo#266