Improve analysis nbsearch grid mapping

Improve analysis nbsearch grid mapping

Now the analysis neighborhood search implements its own version of
put_atoms_in_triclinic_unitcell().  While computing the index of the
correct grid cell, it is relatively easy to produce also the coordinates
that lay within that cell instead of using a separate call.  This
provides two benefits:
 - It avoids rare rounding problems if put_atoms_in_triclinic_unitcell()
   would put the atom right at the edge of the box, but the mapping code
   would consider it outside the box, causing out-of-range grid cell
   index to be generated.
 - It allows to customize the grid mapping more freely (e.g., to create
   grids that are not periodic).

Backported from master with minor changes, fixes #1611.  Kept commit
message the same; the second point will be only relevant for master.

Change-Id: Ib7602fa49a1b8f7882a63843322786b3e51e8e32
(cherry-picked from b3e2e82 in master)