v0.15.0

• Query
  • Removed QueryNames, no longer needed
  • Allow multiple subqueries (annotation/stam#28)
    • subquery() is now subqueries() (iterator).
  • Implemented support for custom attributes on queries and constraints (used e.g. by stam view in stam-tools)
  • Allow DATASET as primary constraint for KEY result type
  • Implemented OPTIONAL qualifier for SELECT subqueries
  • Added Query.bind_from_result() method to bind variables from a previous result
• DataValue/DataOperator: fixes in parsing disjunctions
• DataValue/DataOperator: allow matching some distinct types if the reference is a string
• Implemented substores; allows including other (stand-off) annotation stores as dependencies, keeping STAM JSON serialisations apart (annotation/stam#29)