Non-periodic nbsearch with grid based on bounding box

For cases where there is no full PBC, the analysis neighborhood search
routines now allow constructing the grid based on the bounding box of
the reference positions.  This frees the caller from the need to provide
a reasonably sized box even without PBC to benefit from the grid
searching.

Change-Id: Ia14b7c584a6ca84a5f698c12dc1491dde78bfe6d