Converter21

There is a new MEI importer in this release, as well as many improvements to Humdrum import/export.

Remove code that treats 'Q' as equivalent to 'q' in Humdrum files.

Support Humdrum parsing and writing of RepeatBrackets (e.g. first and second endings). A few unrelated bug fixes.

The build process messed up the un-capitalization of the various package/filenames. This is the same as v1.3, but with the capitalization correct.