Session abstraction in Python module

[eirrgang]

launch() is currently a method of gmx.core.MDSystem and returns a gmxapi::Session.
MDSystem objects are obtained from gmx.core.from_tpr(). They also provide add_potential().
gmxapi::Session objects are exposed as gmx.core.MDSession and provide run() and close() methods.

The simplest demonstration of the need for more abstraction is to represent a session with no work to perform, such as an unallocated MPI rank when the size of the trajectory ensemble is smaller than the MPI context.