Port version of nam_getNamedInstancePSName to python

[skef]

When hinting instances psautohint issues log messages about particular fonts (when using a "reference" font together with other masters) or instances (when hinting a variable font). In the python prototype I'm currently just using "Model %d instance %d" as the instance name where the first number is the vsindex.

Normally one would expect each instance with a glyph to be among the named instances in fvar, but there may be cases where one is missing. Ideally we would have a python-level function (based on fontTools) that returned:

1. The name of the instance, if it is in fvar, or
2. A string with elements corresponding to each axis in order, using the name of the position on that axis in STAT if present and a user-space value otherwise.

For whatever reason fontTools doesn't have much plumbing for this at present. I didn't even find an "unnormalize location" function to go from normalized space back to user space. So this might also be something to add to fontTools and use from there.