Basic support for 2D neighborhood search for analysis

Add support for doing a neighborhood search based on distances in the
X-Y plane into the analysis neighborhood search routines.  The basic
logic is lifted from 'gmx rdf -xy' and made a bit more general.

Currently, only basic searching is implemented, but grid-based should
not be too difficult either.  Later implementation of a grid-based
search should be transparent to callers.

Change-Id: I635d7f7a5eb0136d7a3a4c968ddaa34b03ae3bc7