Remove the set time_future

Per one of our paper's reviewer's suggestion, this commit removes the
unnecessary set 'time_future'.  In a follow-up commit, I'll rename
time_horizon to time_future to match the paper.

Along the way, I removed a couple of unused vintage* sets.
Rename 'time_horizon' to 'time_future'.

This follows on the previous commit, which removed time_future from the
code, per our paper reviewer's suggestion.
Provide a cleaner error message.

For the first (and simple) case of a typo or misunderstanding of the
integer-only requirement of the time_exist and time_future sets, this
commit moves the integerness check to our validation routines.

A number of folks have noted to me that errors produce "impenetrable
output".  This is because most Temoa errors currently come with a long
traceback.  For coopr_python invocations, if the precise error can be
determined, a very specific error message doesn't need the traceback.