Single core lib shared between different problems

[PierreMartinon]

Review the build process (exec / wrapper) and use a single core lib shared for all problems instead of recompiling the whole source each time

• linux is fine, core lib works for both cli execs and python swig wrappers. Significantly faster test execution :-)
• macos seems ok after adding the linker flag "-undefined dynamic_lookup" in addition to the already present "-flat_namespace"
• windows currently fails to build the core lib (LNK1120: 9 unresolved externals on the problem.cpp functions, similar to the error previously encountered on macos)