typo fix
aggregate: replace stupped evaluate with NotImplementedError

When I read the comment I was confused as to how that could be the
correct behavior. But actually, this function is never called by the
tests. Replace the stub with a NotImplementedError, which I think makes
more sense.
groupby: rename fields as prep for UDAs
myrialang: support new CountAll aggregator

Do not have to pretend it reads -zsh instead.