Consider splitting Python package

[LourensVeen]

The muscle3 PyPI package currently installs both the libmuscle and muscle3 modules, with muscle3 having additional dependencies. muscle3 isn't needed by models, and to minimise the potential for version conflicts it would be nice if we had a separate libmuscle package that had a minimum of dependencies. Apparently there are EasyBuild issues too (although EasyBuild does have recent matplotlib packages).

I do remember putting the manager into libmuscle because it was easier that way, but we should have a look at this again and see if we can extract it back out. Or leave it in and just have a dependency of the muscle3 package on the libmuscle package.