Move MyMPILib to NEO-2

[krystophny]

Decision

Move extra/MyMPILib out of libneo and into the NEO-2 repository.

Context

From the dependency audit (#258):

• MyMPILib is only used by NEO-2 (single consumer)
• It adds an MPI dependency to libneo that most downstream codes do not need
• Moving it to its sole consumer simplifies the libneo dependency tree

Action

1. Move extra/MyMPILib/ to the NEO-2 repository
2. Remove add_subdirectory(extra/MyMPILib) and the LIBNEO::MyMPILib target from libneo
3. Update NEO-2 CMake to build MyMPILib from its own source tree instead of pulling it from libneo

PRs

• libneo side: [codex] Remove MyMPILib from libneo #259
• NEO-2 side: Move MyMPILib into NEO-2 NEO-2#66

Ref: #258