Fix to enable partial support for models that have more outputs than
they have inputs, at least in the most basic cases.  This sort of works
the same as how models with more inputs than they have outputs work,
which is that each input is not necessarily directly tied to each
output; instead all outputs are just assumed to have the same
dimensions, which are determined from the result of broadcasting all
inputs with all parameters.

In the future there may be more ways for individual model
implementations to control this (besides outright overriding
prepare_inputs and/or prepare_outputs).

This provides an alternate to this PR:
https://github.com/embray/astropy/pull/12
Add changelog entry.
Merge pull request #3250 from embray/modeling/handle-more-out-than-in

Modeling/handle more out than in