Added specific groupsOfNSliced() and groupsOfNIter() alternatives in sequences.
Added specific groupsOfNSliced() and groupsOfNIter() alternatives in sequences.

The groupsOfNSliced() method is particularly useful for iterating over Django
querysets, but since they don't currently provide a __getslice__ method,
we need to call this variant of groupsOfN() specifically.

larsyencken/cjktools,larsyencken/cjktools