Proper C++ API for analysis neighborhood search.

Replace the dummy C++ wrapper with a more C++-like API for the
neighborhood search routines that are part of the selection code.
Subsequent commits will implement a few additional features to the API,
but split those off from here to keep things easier to review.

Implementation underneath is still the same as earlier.
Will remove the old C functions in a separate commit.
Changed the free volume tool to use the C++ API in preparation for this.

Added some basic tests for the functionality using the new API, which
should increase the percentual coverage in this part of the code
significantly.

Part of #866 and #651.

Change-Id: I139866b82cb048f1676d187b0cf6e5d270d4d32b