Fix triplets and duplets (the normal-notes value was wrongly set to 1 rather than the correct value), fix indentation

-) Store the current rhythmic model in a global variable, loop index variable
-) Factor out raw duration and notehead functions to parse values also for
   rhythmic models
-) Reset rhythmic model as soon as a new explicit duration is encountered

Otherwise tuplet handling will need to explicitly check for a beam start...

Implement all time signatures with appropriate fallbacks and warning if feature not supported by MusicXML

Cache all alteration for the whole current measure, so that xCC displays e.g. two Cis. Need to reset after each measure

-) For mensural clefs, write out a warning that MusicXML does not support them
-) Move all clef-generation to a dedicated clef (..) function
-) Also implement g-2 clef (tenor clef)

Add -f cmd-line param to distinguish RISM files and pure P&E files (without any metadata, just incipits)

Gracefully handle invalid space characters inside the P&E code (print a warning, but continue processing)