Move nbnxn files to nbnxm directory

Move nbnxn files to nbnxm directory

This change moves all files related to the nbnxn code to an new
nbnxm subdirectory. This change is only code motion and file renaming.
This is also the start of a renaming from nbnxn to nbnxm, which is
a more correct name, since the i- and j-cluster size can be different.
All nbnxn setup code in forcerec.cpp is moved to nbnxm_setup.cpp.
Removed nbnxn_ and _simd... pre- and postfixes from most files.
Renamed nbnxn_search.cpp to pairlist.cpp and nbnxn_search.h to
nbnxm_pairlistset.h. Although this is currently somewhat inconsistent,
future refactoring will make the contents consistent.
Renamed nbnxn_utility.h/cpp to nbnxm_geometry.h/cpp.

Change-Id: I39c17769c566f78b84211510c5ac17793fd131fa