Refactor to use lowercase namespace

[Matthew-Whitlock]

Done to match the typical standard for namespace capitalization, but also to avoid a specific name clash nuisance in the Fenix Lammps work (which has a Fenix class, to follow their naming conventions).

fenix->fenix_rt and Fenix::->fenix::

Explaining 66 more deletions than insertions:
61: Deleted globals.cpp by declaring fenix_rt as inline
1: Remove globals.cpp from src/CMakeLists.txt
2: Remove some excess whitespace from fenix_ext.hpp
2: Remove whitespace and debug print from fenix_opt.cpp

Otherwise insertions/deletions are balanced as expected.