Mutatormath

- Fixed problem caused by files in which a source element has no "name" attribute, by adding a temporary simple unique name (80).
- The designspace document description now lives at the designSpaceDocument repository: https://github.com/LettError/designSpaceDocument.
- Pass UFO groups to the MathKerning object so that the latter is able to identify the exceptions (82).

No major changes compared to master, but for some consumers it is good to have a release. So here it is.