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

Gen/AmpGen/AmpGen/Expression.h

@@ -37,9 +37,9 @@
 #include "AmpGen/MsgService.h"
 
 namespace AmpGen { 
-  class Expression;
-  class Parameter;
-  class ASTResolver; 
+  struct Expression;
+  struct Parameter;
+  struct ASTResolver; 
 
   /**@class IExpression 
    *   Virtual base class for other expression tree components.