Parse `instanceInterpolations` values correctly

Montserrat has some [`GSInstance::instanceInterpolations`](https://docu.glyphsapp.com/#instanceInterpolations) with coefficients that look like `NSPoint`s rather than floats, e.g.

```
instanceInterpolations = {
UUID0 = "{0.777778, 0.733333}";
"1C84A7F3-C40A-4DC1-B8B4-102D183C348F" = "{0.222222, 0.266667}";
};
```

The fix for #238 parses those values as strings, but we should probably define more precisely what are the possible values of `instanceInterpolations` and parse them accordingly, maybe as points.

@schriftgestalt

Review the parsing change from #238 and the handling of GSInstance::instanceInterpolations. Determine the valid value shapes, including the shown NSPoint-like strings, and define parsing that preserves them appropriately. Done means instanceInterpolations values are parsed according to their supported forms rather than uniformly as strings.

Written by the indexing model from the issue text.