implement Music21Loader

[johentsch]

• strip superfluous columns from facets
  • events shadow structure table (systematic use of **higher_level_info)
  • make a selection of columns replacing calls to parse_m21_object() (its use of getattr on m21 objects seems to slow parsing down, also)
• harmonize column names with ms3 wherever possible/practical
• clean datatypes, e.g. columns containing ints and NaNs are currently floats
• propagate values such as time signatures or decide that the FeatureExtractor takes care of that