AmpGen into AmpGen development branch

These are fixes for AmpGen, mostly focused on the stand-alone build system, LLVM compatibility, and macOS support.

Fixes:

• Lots of CMake fixes, adds symbolic links to options files
• Printout fixes, debug can be activated from CMakeLists_local
• CLI11 is used for Generator, same syntax but now supports -h and fewer segfaults
• OpenMP is now optional
• Print statements now flush, causing the final error message to be printed on a crash
• Lots of cleanups from LLVM, such as missing includes, mismatched definitions, invalid definitions
• prob_unnormalised was missing from FixedLibGenerator