Add an ndim parameter to one of the array_iter specializations

This is to allow one dimensional iteration to quick-fix a bug
in the groupby evaluation. The role of array_iter within needs revisiting
as it is specific to strided arrays.
Fix bug in groupby of multi-dimensional input

This is because , is an outer level thing in datashape, which
dynd's types are a superset of.
Use ; instead of , to separate printing of dynd types