MusicXML: Add possible \layout block, use autoBeaming=##f by default, allow manual...

MusicXML: Add possible \layout block, use autoBeaming=##f by default, allow manual beaming

-) Added a musicexp.Layout class to print out a \layout block. It contains a dict
   ContextName<=>ListOfEntries.
-) Added a global variable to hold the layout options, so whenever we encounter
   the need for an entry in the \layout block, we can simply add the corresponding
   setting.
-) By default, autoBeaming should be ##f (since MusicXML files contain full beaming
   information), unless beaming should be ignored (e.g. for wrong musicxml files, like
   the ones from Sibelius). I'm already using the Layout class for this.
-) Add command line option to ignore beaming information in the MusicXML file and
   use lilypond's autoBeaming.