Provide callbacks/notifications for MDModules

Provide callbacks/notifications for MDModules

Adds functionality for MdModules to subscribe to be called back during
the simulation. Within the run, subscribed modules are notified of
events, that are distinguished by the function argument by the call back
function.

Implements the callbacks based on storing function pointers, following
the discussion in https://gerrit.gromacs.org/c/gromacs/+/10942

refs #2945

Change-Id: I61589215fa9beb79825f0b5261ed50b4116046ff