Decouple task assignment from task execution

Decouple task assignment from task execution

Code that needs to run on a GPU does not also need to know about the
code and data structures that underpin task assignment.  The outcome
of task assignment is the information about which GPU to use, and it
is simple and effective to give just that result to the code that
needs it.

Simplifies t_forcerec.

Added more const correctness for gmx_device_info_t pointers.

Change-Id: I094c19e08be73af998bd287e43d5c2b6e5969a60