instead of having pair_dpd_fdt* make the SSA-neighbor list request for it.
Forces an "extra" list to be built, but now skip lists work properly.
Maybe we can detect if skip lists won't be used, and squash the extra list.
Includes documentation and examples.
NOTE: VV requires very small timesteps under isoenergetic conditions.
Consider using fix_shardlow instead, since this VV support is
primarily for comparison purposes.
