Default unison label should be P1 or 1, not '_'.

[alexandermorgan]

The default label of a horizontal unison in a given voice currently defaults to '_' when the "unison" arises from a single sustained note (as opposed to a reiterated note). This default has the effect of distinguishing between events that users would usually want to be aggregated together so the default should be changed. The use of 'P1' or '1' labels for the unison should be determined by the user's settings, specifically whether they do or do not choose to show interval quality.

[crantila]

Thanks---nice work! I'll release a new version of vis-framework to the PyPI.