Fixed: Slightly optimize loop

Instead of checking to see if an object is a Line, only iterate over the Line objects
Fixed: Slightly optimized Volta bracket encodings

Removed a couple loops over all the objects so hopefully it's a bit faster now.
Fixed: Notes and Grace Notes could have the same ID

Since Grace Notes can appear at the same position as regular notes they could be given the same ID. This commit fixes it so that grace notes will have a unique ID.
Fixed: Clef handling is now compatible with MEI 2013
New: libmei methods for working with child elements
Fixed: sibmei_batch will work with musicxml files
Fixed: StaffDef/StaffGrp now creates valid MEI