mx::core Support a newer version of MusicXML

[webern]

We are stuck on MusicXML 3.0 because it’s a big job to upgrade to 3.1 (or whichever version is out when we get to it).

Do this without breaking mx::api

• [x] Create a program that parses the XSD. (In Progress)
  • [x] parse xs:annotation
  • [x] parse xs:attribute
  • [x] parse xs:attributeGroup
  • [x] parse xs:choice
  • [x] parse xs:complexContent
  • [x] parse xs:complexType
  • [x] parse xs:documentation
  • [x] parse xs:element
  • [x] parse xs:enumeration
  • [x] parse xs:extension
  • [x] parse xs:group
  • [x] parse xs:import
  • [x] parse xs:maxInclusive
  • [x] parse xs:minExclusive
  • [x] parse xs:minInclusive
  • [x] parse xs:pattern
  • [x] parse xs:restriction
  • [x] parse xs:schema
  • [x] parse xs:sequence
  • [x] parse xs:simpleContent
  • [x] parse xs:simpleType
  • [x] parse xs:union
• [ ] Create a program that flattens and links the XSD.
• [x] Create a program that converts the XSD into programming friendly types (Class, Enum, etc).
• [x] Regenerate Enums
• [ ] Regenerate other SimpleTypes
• [x] Regenerate unions
• [ ] Check for changes to custom simple types during generating.
• [ ] Regenerate Attributes
• [ ] Regenerate Choices
• [ ] Regenerate Groups
• [ ] Regenerate Elements
• [ ] Create a program that re-generates mx::core for MusicXML 3.0 without too much breakage.
• [ ] Use the program to generate mx::core for MusicXML 3.1 (or 3.2 or 4.0, https://github.com/w3c/musicxml/issues/314) without breakage.
• [ ] Expose some new functionality from mx::core to mx:api.