Don't assign measure number 0 to incipits

[johentsch]

Incipits should be considered as markup, not part of the music. Right now they are excluded from bar count and get MN 0 but that makes them indistinguishable from upbeat measures. Suggestion: Replace 0 by x or missing value

[johentsch]

This issue is postponed until a measure_number is added which allows for including string values such as 16b. This will also enable a special value for incipit measures.

For now, there are a couple of other ways to mark/discover incipit measures:

• one could have a convention where N incipit measures are encoded by adding -N to the measure count of MC 1 (rather than setting "Exclude from measure count").
• the presence of an tag is a strong indicator, in particular in combination with
• the absence of tags (does not work if the incipit actually includes the incipit, i.e. notes)
• but the most fail-safe method, actually, might be to look for a tag after one of the first tags because this what's used to separate the incipit from the actual beginning