Fixes for mensuration, coloration, ligatures, and keySig

[martha-thomae]

This pull request fixes issues related to: mensuration, coloration, ligatures, and key signatures.

• Mensuration

  • Fix retrieval of modus minor value from the MEI file (it was due to a small typo).
  • Add support in the interface for entering not only the mensuration values (modus, tempus, and prolatio), but also the mensuration sign if present.
  • Also allow for mensuration to be encoded according to the most up-to-date version of *mens (using `met(X)_####, whereXis the mensuration sign and####` stand for the maximodus, modus, tempus, and prolatio values).
  • Fix encoding of these signs in MEI (by erasing code that removed @sign and @dot).
  • And parse the mensuration sign from an uploaded MEI file.

• Coloration

  • Add retrieval of coloration from MEI files.
  • Add support for entering colored rests.

• Ligatures

  • Fix parsing of ligatures that end on a dotted note when uploading an MEI file.

• Key signatures

  • Fix issue regarding key signatures entered by the user (and present in Humdrum), not being saved in the Part-based MEI file.

[JRegimbal]

This all seems good to me! If you've gone through and things are working feel free to merge.

[martha-thomae]

Thank you! :) This is the first time that I do a rebuilt of the GitHub pages version of the website, but I did follow the instructions in https://github.com/MeasuringPolyphony/mp_editor#rebuilding-the-github-pages-version. I think it looks ok.

Oh, btw, I cannot merge, I don't have "write access" to this repo.