Analysis grid nbsearch for more cases

Add support for grid neighborhood searching also for epbcXY and
epbcNONE, as well as the XY mode in the analysis neighborhood search.
The grid is done based on the input box size also for the non-periodic
dimensions, or a single grid cell is used if the box size is unknown in
that dimension.  In the future, an option could be added to determine
the grid automatically from the bounding box, but that is outside the
scope of this change.  For the XY mode, a single grid cell is always
used in Z.

Improved the logic of determining when grid search is possible and/or
reasonable and implemented the option to force the grid
searching (since the test data currently would not trigger it
otherwise).

Change-Id: I0c43bf95d7d5dd46e57923cdf0d6864fbb3dfe59