Updates measure numbers

To make the measure numbers displayed in the scores comply with a wide-spread convention in music printing, all anacrustic measures have been systematically excluded from incrementing the counter. Likewise, second endings have been adapted so that their measure numbers comply with the a)-b) convention (i.e., in general, first and second have the same measure number).

A new column in the README table (Measure numbers) marks those pieces for which one or several measure numbers changed compared to the previous version of the corpus.