Feat: Use MorphIO by default and fallback to internal loader in case of failure

[adrien-berchet]

requires https://github.com/BlueBrain/MorphIO/pull/431

This is a draft to use MorphIO as default loader. But for now MorphIO silently fixes the loaded morphologies according to arbitrary (though relevant) rules (as described in https://github.com/BlueBrain/MorphIO/issues/427), which is an issue for TMD. So we should update MorphIO to be optionally less strict before making it the default loader.