Split nbnxn pairlist types for CPU and GPU

Split nbnxn pairlist types for CPU and GPU

Split nbnxn_pairlist_t into NbnxnPairlistCpu and NbnxnPairlistGpu.
Now all members are used in each struct in the respective code paths.
Templated make_nbnxn_pairlist_part on pairlist type.
This change is only refactoring, except for changing an assert
to a release assert.

Note that currently the two list types have some members in common,
but most of those will dissappear in followup changes.

Change-Id: Ic53585066874c4a42539a1b1bc68b756dda3f19e