Change nbnxn_search to class PairSearch

Change nbnxn_search to class PairSearch

Changed some nbnxm functions to methods located in new file nbnxm.cpp
to keep the internals of the nbnxm module private and hidden.

TODO: Rename internal.h to pairsearch.h
TODO: Move Pairsearch definitions to new file pairsearch.cpp
TODO: Rename nonbonded_verlet_t to Nbxnm

Change-Id: I8fa5d3b88ed453598cbed3ab4b8548258f851e20