New layer improvements/bug fixes

[pettter]

If a staff is empty of notes (in particular in its first measure) this gives an error, due to the computation of the pitch grid depending on finding the center line of the staff and a note with a known pitch to give the right offset. This should be fixed by

• Removing empty staves
• Calculating the right offset using the clefs rather than random notes. This isn't necessarily easy, because the IDs don't seem to match with the MEI for some reason, but I think the symbol IDs are consistent, so we can find the clef and it's height, and compare that to the height of the middle line of the staff, in theory.