User/design space mapping computation: Improve default generation?

In the BraceTestFont.glyphs file, the generated axes mapping is

```XML










```

If one removes the "Axis Location" custom parameters from the masters, one gets

```XML






```

Looking at the instances, glyphsLib should be able to infer that we need to add a mapping that maps user 75 to design 50 and user 100 to design 100 ("Condensed" instances). Likewise, we have two weight masters and define weightClass 100 and 700 in the instances as min/max. Basically, glyphsLib should infer the first axes definition without the user setting the appropriate custom parameters.

Start with the generated axes mapping in BraceTestFont.glyphs and compare it with the instances, including their weightClass values and the absence of Axis Location custom parameters. Done means glyphsLib infers the user-to-design mappings for the condensed and weight axes, producing the expected axes XML without those custom parameters.

Written by the indexing model from the issue text.