Read/Write smart component intermediate layer

[ollimeier]

          > > * In Glyphs, smart component base glyphs can not have intermediate masters. When I add one in Fontra, I get no error, but the data gets messed up, and if you reload the "intermediate" has taken over the default, and there isn't really an intermediate. I think putGlyph should raise an error in this case.

About this bullet point: It is possible to add an intermediate master in a smart component (within GlyphsApp):

But I have to look into the code how fontra converts it into the GlyphsApp file.

@justvanrossum This topic is a bit more complex. I tried to find a solution and I got quite far, but at the end I had the feeling I am wasting time, because this 'Intermediate layer' in a smart component is definitely an edge case. I would prefer to prioritize other things and see this 'smart component intermediate layer issue' as a follow up probably after Version 1.0? What do you think?
But as you suggested in the first place, I implemented an error message "NotImplementedError".

Originally posted by @ollimeier in #76 (comment)