`FbcReactionPlugin::createGeneProductAssociation` is not exposed in C API

[exaexa]

When working on SBML.jl I just noticed that the function that would allow straightforward adding of GPAs to the Fbc reactions is not exposed. I assume FbcReactionPlugin::createGeneProductAssociation should be available for C API as ``FbcReactionPlugin_createGeneProductAssociation`. This prevents creating GPAs reliably from C.

I'll write a PR later today if I have time (hints on how to autogenerate the doxygen docstrings etc welcome. :] )

-mk