Adds breve notes

A project I'm working on required breves, so I added those as a supported note duration. I used the duration of 0 to specify breve.
why not 1/2 ? if Breve = 0 what would a Longa (=2 breves) or Maxima (=4 breves) be?  Humdrum uses 0, 00, and 000, but I find that messy compared with "1/2", "1/4", "1/8".