Feature: multiple target files for code generators

[floooh]

It should be possible to define multiple source and header target files for the code generator:

fips_generate(... SOURCES Source1.cpp Source2.cpp HEADERS Header1.h Header2.h) 

This would also require a new entry point in the python-generator-scripts which accepts arrays for source and header files, to be backward compatible with existing generators.

See here for a real-world use case: #131