Fix too small GPU pair count estimates

For triclinic unit-cells with DD the non-local cluster pair count
estimate was too high, especially for thin local domains, due to an
incorrect estimate of the cluster size. Since the pair count estimate
for the local pair-list was determined as a total minus a non-local
estimate, the local estimate could get negative and cause exceptions.
Fixed the cluster size estimate and added a lower limit for the local
size estimate.

Fixes #1762.

Change-Id: I3489550968f66bc03ba4e6056017a58eba37f7cc