Single core lib shared between different problems

Review the build process (exec / wrapper) and use a single core lib shared for all problems instead of recompiling the whole source each time
- [x] linux is fine, core lib works for both cli execs and python swig wrappers. Significantly faster test execution :-)
- [x] 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)