kicad-parse-gen does not recognize solder_paste_margin

[seppestas]

When parsing a KiCad layout that contains a solder_paste_margin, kicad-parse-gen fails with the following error:

ERROR: Kicad Symbolic Expression Error: symbolic expression other error: unknown element in module: solder_paste_margin.

This element does not seem to be defined in the file formats spec, but it can be found in the list of PCB keywords.

[adamheinrich]

Maybe we can cherry-pick commit a842a52 from branch kicad5.