add FbcReactionPlugin_createGeneProductAssociation to C API

[exaexa]

Description

This simply adds a C wrapper for the C++ method.

Motivation and Context

We needed the wrappers for making reactions with GPA from SBML.jl.

Closes #246 .

Cc @giordano .

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically

[fbergmann]

thank you!