Modularize libmambapy

[AntoinePrv]

• Modularize libmambapy
• Add test-libmambapy to Taskfile

[JohanMabille]

I find it confusing to have the file main.cpp in the core subfolder (implying it's not the main for the whole library), defining the bindings, and having the main macro PYDBIND11_MODULE called in the file core.cpp file. It looks to me like the logic is inverted.

Even if we split the bindings in many files (which I agree is far better than a huge monolithic file), we will keep a single global module (libmambapy), and the name passed to the PYBIND11_MODULE macro should be the same as that of the extension library (dll/dylib/so) to avoid this issue (although the tests seem to pass, so I might have missed something).

A more natural code organization would be one pair of hpp/cpp per libmamba subfolder that we want to bind, and the main.cpp defining the module (i.e. calling PYBIND11_MODULE) and calling the bind_submodule functions defined in the other files.