Leveraging View.iter() & extended metadata

• changes to iter methods for iterating through DataFrames and metadata belonging together:
  • supressed the second item: instead of (metadata, paths, df1, df2...) yield (metadata, df1, df2...) where the
    metadata dict contains the paths
  • added methods iter_transformed() and iter_notes() to Parse and View objects
• added command ms3 transform
  • used to concatenate all parsed TSVs of a certain type into one file including the option to unfold and add quarterbeats
  • stores them with prefix concatenated_; ms3 now ignores all files beginning with this prefix
• changes in default TSV columns
  • metadata.tsv includes the new columns
    • length_qb: a scores length in quarterbeats (including all voltas)
    • length_qb_unfolded: the same but with unfolded repeats, if any
    • all_notes_qb: the sum of all note durations in quarterbeats
    • n_onsets: the number of all onsets
  • no empty volta columns are included (except for measures) when no voltas are present