Build variable fonts for Google Fonts

[simoncozens]

This PR allows for the existing fonts to be built as variable fonts using a libre toolchain, a requirement for upgrading the fonts on Google Fonts. This was done by:

• Inverting the axis mapping in the .glyphs source file, as detailed in Problem with weight axis in variable font #73.
• Developing functionality within the babelfont font source translator to "bake in" Glyphs custom parameters such as "Remove Glyphs" and "Replace Features".
• Automatically generating transformed versions of Cormorant Infant, Cormorant Unicase and Cormorant Garamond sources using babelfont
• Compiling these generated sources with gftools-builder. A Makefile is included which sets up the Python virtual environment, rebuilds the sources files and calls gftools-builder.

Note that we store the generated sources in the repository so that the fonts can be recompiled at a later date only using a builder process with access to the repo contents.

Will be used to fix google/fonts#8449.