Add ability for 'datafilter' command to modify data sets.

[drroe]

Previously, the datafilter command only generated sets with 1 for cutoffs satisfied and 0 otherwise. Now cpptraj has new keywords that allow one to modify a data set to only contain data satisfying the cutoffs:

filterset <set> [newset <newname>]
  If 'filterset' is specified, the specified <set> will be modified
  to only contain '1' frames; cannot be used with 'multi'. If 'newset'
  is also specified, a new set will be created containing the '1' frames instead.

This currently only works for scalar 1D sets. Test added and manual updated.

[swails]

run jenkins