Improve NeighborLists by pruning and dynamic sizing

[kuberry]

pruning:

Provide a mask to the NeighborLists object which has the same dimension as the number of target sites.

Based on this mask, remove neighbor lists for targets that are marked for deletion.

This pruning is not meant to be incremental (done all-at-once)

insertion:

Add a parameter to neighbor lists indicating allocation should be provided. If attempt to insert a number of neighbors for a new target fails, then allocate using same rules underlying std::vector.

This functionality would be useful for #211