payasan - towards removing SectionInfo

Today's commit was trivial because I'm working up to removing SectionInfo but I'm reluctant to do it without spending time designing a replacement with pen and paper.

SectionInfo should be removed because it is indiscriminate and labels Sections with misleading information - Sections in percussion notation always stored (but didn't write) key information. All Sections stored BPM and (ABC) Unit Note Length regardless of whether they used the MIDI or ABC backends.

I'm not sure how to do this - adding another parameter to the Section type is unattractive, it is already polymorphic on pitch, duration and (LilyPond) annotation. As Sections are named, section info could be stored in a map in the header of a Part but this "disperses" information away from where it is created. The latter may provide a means of having independent section info for multiple backends, though.