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

• Create a program that parses the XSD. (In Progress)
  • parse xs:annotation
  • parse xs:attribute
  • parse xs:attributeGroup
  • parse xs:choice
  • parse xs:complexContent
  • parse xs:complexType
  • parse xs:documentation
  • parse xs:element
  • parse xs:enumeration
  • parse xs:extension
  • parse xs:group
  • parse xs:import
  • parse xs:maxInclusive
  • parse xs:minExclusive
  • parse xs:minInclusive
  • parse xs:pattern
  • parse xs:restriction
  • parse xs:schema
  • parse xs:sequence
  • parse xs:simpleContent
  • parse xs:simpleType
  • parse xs:union
• Create a program that flattens and links the XSD.
• Create a program that converts the XSD into programming friendly types (Class, Enum, etc).
• Regenerate Enums
• Regenerate other SimpleTypes
• 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, Should the next version be 3.2 or 4.0? w3c/musicxml#314) without breakage.
• Expose some new functionality from mx::core to mx:api.