Remove nbnxm kernel type from pairlist generation

Remove nbnxm kernel type from pairlist generation

The type of kernel for computing distances between atom clusters pairs
is no longer set directly by the non-bonded interaction kernel type.
A new type is added for this, although the actual choice of cluster
distance kernel type has actually not changed.
Also moved the declaration of PairlistType to pairlistparams.h.

Change-Id: Iaf38ca7804eed75295e6cb0e1176a23d620c6f0c