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.
Merge pull request #83 from mabraham/opencl

Refactor JIT support