Convert nbnxn_pairlist_set_t to class PairlistSet

Convert nbnxn_pairlist_set_t to class PairlistSet

This change is only refactoring.
Two implementation details have changed:
The CPU and GPU pairlist objects are now straight lists instead
of array of pointer to lists. This means that the pairlist objects are
no longer allocated on their respecitive thtreads. But since the lists
have buffers to avoid cache polution and the actual i- and j-lists are
llocated thread local, this should not affect performance
ignificantly.
The free-energy lists are now only allocated with perturbed atoms.

Change-Id: Ifc76608215518edfc61c0ca8eb71ea2a928cf57c