Export wav using fluidsynth

[CarlosCancino-Chacon]

This pull request adds the following functionality:

• Export a partitura object to an audio file using Fluidsynth.

  • This functionality is available in pretty_midi, where it only works for MIDI files.
  • Such a method is useful for cross-modal tasks (e.g., transcription, audio-to-score alignment, etc.).
  • This implementation allows for the method to work on all partitura note-based objects (Score, Performance, Part, PerformedPart, note arrays), and thus for all formats supported by partitura.
  • Fluidsynth is treated as an optional import. Instead of adding to the repository a default soundfont (as is the case in pretty_midi, which includes a very small one), the default soundfont is set to be downloaded from the MuseScore soundfonts (which are distributed with the MIT license).

• Fixes a few minor issues in slice_ppart_by_time adding previously missing MIDI messages and fixing the computation of onset and offset times in ticks.

[manoskary]

A very nice addition. This PR adds export wav with fluidsynth. It includes some useful utilities such as downloading a file.

[manoskary]

Nice addition, a few comments and questions were already addressed and answered.