Rename beta1d and beta2d - solves issue 3029 (This takes over #3102)

This commit follows on a rebase, where I rebased the work of #3102 on the
current master. Thus, this closes #3102 which in turn fixes #3029.
    
@astronomeralex You did a great job in #3102 and if I had not screwed up the rebase
(I deleted the wrong line in a merge conflict) then my name would not turn up here
at all. I've based this on your branch and github will recognize you are the author
for your commits.
As you may have observed, it is very uncommon in astropy for other people to take ov
and finish a pull request that they did not initiate themselves, but I want to make
sure that this makes it into the 1.0 release as it will be much harder to change the
name of a model once 1.0 is released.