Ignore MIDI files in MuseData to Humdrum coverter.

[craigsapp]

In the file https://github.com/musedata/corelli/blob/master/op1/musedata/corelli-op1n10-03.msd#L889-L918
there is a file used for MIDI conversion. This file should not be treated as an instrumental part as is currently being done.

These lines would be used to detect if it is related to MIDI:

Group memberships: midi
midi: part 1 of 4

(but there is only one file not four as implied by the "part 1 of 4" text. Alternatively, any filename that has "mchan" in it should be ignored:

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
FILENAME = mchan2
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&