Feature (MusicXML):  parse <extend/> tags in figured bass

Added the <extend/> tag to the library of MusicXML tags, and created the appropriate object class for it.

Note: Currently, the tag will only be read if inside a <figure> tag. For functionality in other instances, simply add another
“elif self._mxObjs['otherTagName'] is not None:”
to xmlHandler.py
Update (MusicXML): rewrote <prefix> and <suffix> tag handling

Rewrote code to be more elegant and efficient. No change in functionality