Renaming baseMoment

[Dan Eble]

To follow up the replacement of make-moment by \musicLength [1], I think 
it would make sense to rename baseMoment to beatBaseLength.  Are there 
any concerns or other suggestions?
--
Dan

[1] 
https://gitlab.com/lilypond/lilypond/-/commit/73ea27187caadc4828d99226f0be64035760fe42