fix: deprecatge PyPI package

[sbillinge]

Problem

diffpy.snmf exists on pypi but this is no longer used.

Proposed solution

Deprecate it. It is not allowed to simply remove packages, so follow PyPi protocols for how to do this. Basically, have it point to a legitimate version of the package but at the same time provide a deprecation message, something like that.

I didn't check if it is also on conda-forge with the old name, but if it is we have to do the same work over there (though the process may be different)

We need to do the same deprecations for any documentation that exists under the old name.

@john-halloran please can you look into this?