Pip package for distribution #92

To simplify distribution we want to distribute the compiled code within pip packages.

Models should also be part of the pip package.

We need to differentiate between packages for internal use (not on pypi) on package for external use (on pypi).

We need ensure that:

• internal packages contain the required models.
• external packages don't fail because of missing files or non-existing paths

We need to fix the linking of the python extension. Currently it is setup to link to the absolute path of the mujoco lib in the venv, it needs to be relative for distribution.