Avoid unnecessary exclusion generation in DD

Avoid unnecessary exclusion generation in DD

With only self exclusions, the DD partitioning would generate a list
of all empty exclusion lists for all i-atoms. Now no lists are
generated. An assertion is added in the pairlist generation code
to ensure we either have 0 or the right number of exclusions lists.

Change-Id: I4b0556cbf3ed3a6fea7bdd0e03e194410bcfb88a