Relax pull PBC check
The check in the pull code for COM distances close to half the box
was to strict for directional pulling. Now dimensions orthogonal
to the pull vector are no longer checked. The check was actually
not strict enough for directional pulling along x or y in triclinic
units cells, but that is a corner case.
Furthermore, the direction-periodic hint is now only printed with
geometry direction.
Added tests for the maximum pull distance calcuation.
Fixes #1962.
Change-Id: I8e389ba3f0490ca67586fd10bdc9d71d9957ab45