[SCIP] Use .dec file writer from code (SCIP/GCG)

[Christopher G]

Dear SCIP community,

I'd like to use the .dec writer from GCG as defined here [1] in my C++ code to write a `gcg::PARTIALDECOMP` to a .dec file -- either the decomposition that is currently applied to a given `SCIP*` or a given `gcg::PARTIALDECOMP`. How would I achieve that? How do I write to files with methods defined with `SCIP_DECL_READERWRITE(...)` from my code? 

Best,
Christopher G

[1] https://github.com/scipopt/gcg/blob/1ed540d9158490f3109735df93718a4eaf91acc1/src/gcg/reader_dec.cpp#L1318C23-L1318C37