remove attributes from reconstruction staves

not working propely: getAttributes doesn't seem to return the correct
list -> to be investigated.
updated documentation comments
moved chain_elems to utilities
add sourceDesc for variant sources
added utility functions
reconstruction test with two reconstructed voices
add editor element for each editor

move sourceDesc into sources.py
fixing attribute adjustment in placeholder staffDefs
Merge branch 'adjustStaffDefs'

Conflicts:
	massage/test/transform_test.py
Merge branch 'sourceDesc'
Fixed duplicated xml:id