Support subset of rvec array as input for analysis nbsearch
Make it possible to perform a neighbor search such that instead of using
a continuous rvec array as the reference/test positions, it is possible
considers a subset of such an array, specified by a separate index
array.
Also, add support for accessing the components of the shortest distance
vector between found pairs through AnalysisNeighborhoodPair. This frees
the caller from a separate pbc_dx() call in case that distance vector is
of interest.
Tests could still be extended and improved, but left for a later change.
Change-Id: I253ddda483122367d9c41a5ed8bb172e476a90bb