Made cutoff-scheme=Verlet the default

Since the most common options are now supported by the Verlet
cut-off scheme and is it more accurate by default, supports GPUs
and is faster for any non-water system, this is now the default.
Not supported yet are pbc!=xyz and generalized-Born.

Change-Id: Iba292d22c6f3a4dce18d0911b062f2beb70365cb