CMME conversion

[DILewis]

Issues arising from Biblissima-hosted meeting to work on support for the CMME format for mensural notation (see cmme.org)

Some things are not supported or less well supported in MEI:

• [ ] Double-headed bmol (flat) signs [ignored in conversion]
• [ ] Gamma clefs (rare) [ignored in conversion]
• [ ] Half coloration (notes that span colored mensural units) [ignored in conversion]
• [ ] CMME describes barlines in terms of number of lines and presence of repeat dots separately (so single barline with repeats is possible [currently just using single, double, rptboth]
• [ ] OriginalText equivalent not available in the right place
• [ ] Signum Congruentiae are common, but not supported [currently using @type]

Some Verovio issues:

• [ ] Fermatas are not rendered
• [ ] We use <gap> for <cmme:Ellipsis>, but this is not visualised (in CMME, there is a stave gap)
• [ ] Breaks in layers have no effect (for obvious reasons), but could be visualised in other ways

We're also doing some less good things:

• [ ] Not all annotations are preserved

[DILewis]

Also missing from MEI:

• <space> has duration information, but is missing num and numbase, which is needed for mensural durations

[DILewis]

For signum congruentiae, CMME has <Offset>, which is a vertical position. This is used in our sample files, but only to resolve clashes of symbols. Currently we do nothing with this.
Similarly, we ignore Orientation for the Signum (which indicates whether the symbol is flipped, usually for use below the staff). Signum also takes Side, which gives a horizontal offset left or right, possibly to support symbols that happen between notes or rests. Neither of these latter occur in our sample data.