Absorb MyMPILib from libneo, adopt shared species/collision_freqs

[krystophny]

Context

The libneo dependency audit (itpplasma/libneo#258) identified:

1. MyMPILib is only used by NEO-2. The team decided to move it from libneo into NEO-2 (Move MyMPILib to NEO-2 libneo#265).
2. NEO-2 should adopt the shared species/collision_freqs/transport chain from libneo (Group species, collision_freqs, transport as a physics sub-library libneo#266) instead of maintaining any own implementations.

Action

1. Absorb MyMPILib into the NEO-2 repository when it is removed from libneo
2. Update CMake to build MyMPILib from NEO-2's own source tree
3. Verify NEO-2 uses libneo's species/collision_freqs where applicable
4. Remove any duplicate implementations

Ref: itpplasma/libneo#258, itpplasma/libneo#265, itpplasma/libneo#266