Refactor JIT support

JIT compilation was being done earlier than needed, but some
reorganization of the existing Gromacs is needed to move the
compilation to a place where call graphs and module dependencies flow
more smoothly.

Related, some code has moved from data_mgmt to generic nbnxn_gpu
files, because they were not really about data management.
Flavors of  what? GPUs? While I appreciate encapsulation, this function is an overkill: 50 lines of straightforward code placed in an ill named and undocumented function does more bad than good. I'd prefer a name that is less confusing e.g. nbnxn_ocl_map_interaction_types_to_gpu_kernel_flavors() and documentation too.