Eliminate Warnings

https://op-musicology.epfl.ch/projects/distant-listening-corpus/work_packages/1355/

This repo contains an unresolved ERROR message due to an edge case VOLTA scenario which cannot be ignored.

In op23no3 the VOLTAS cannot be computed, because of the unique volta repeat structure. Not only are there voltas of different lengths in this piece, but also the first volta is repeated more than once, so there seems to be two values in the volta_start column separated by a comma to represent this repetition, instead of a single integer value. At the moment I don't know which column is being referred to, since metadata.tsv on my end contains only volta_mcs. I've tried reading up about this, but as of now can't see how to fix it. @johentsch please advise.