Staff propriety in clefs

[fosfrancesco]

There are several elements in partitura that have a staff-like attribute. In particular:

• all subclasses of GenericNote
• all subclasses of Direction
• Words
• Clef

Such attribute is always called '''staff''' except in Clefs where it is called '''number'''.
I guess the initial reason for that was that it is called like that in musicxml. Can I change that? This is not backward compatible and will create problems in all old code using this propriety. But I think this will not be a lot of code and now it is a good time to do it.

[sildater]

sounds good to me. make sure to change it in the relevant i/o functions too.

[manoskary]

+1 for fixing this issue. In either case, I think the next release of partitura should be a proper one then, i.e. version 1.0.0