Ability to specify which instances and master should be converted to designspace

GlyphsLib currently uses all masters and instances to make a designspace file. I've come across many .glyphs files which have experimental/wip instances or masters which should be ignored. It would be nice to have optional params on to_designspace which would allow users to specify what they want.

We also have a situation where our api only supports the wght axis atm so we need to manually split .glyphs files ourselves which is bad.

This may be a niche request so feel free to ignore.

Start at the Python to_designspace entry point and trace how it currently selects all masters and instances. Define optional selection parameters for masters and instances, including the wght-only use case, and verify that only the selected inputs are emitted in the designspace.

Written by the indexing model from the issue text.