Attributes / properties: enable several items in the configuration for a given method name

[nthiery]

Example: the current syntax prevents specifying that "conjugate" should be displayed as a property both for Parititions and Tableaux:

conjugate:
  in : Tableaux()

conjugate:
  in : Partitions()

A potential alternative syntax could be:

- method: conjugate:
  in : Tableaux()
- method: conjugate:
  in : Partitions()

This would also enable separate items for in / not in:

- method: evaluation
  in: Tableaux()
- method: evaluation:
  in Tableaux()
  not when: is_standard

The in-memory representation should be adapted to enable fast query by method name;
presumably a dictionary mapping each method name to the list of relevant items. All items would be evaluated. The result would be True if there is an :in/when" match and no "not in/not when" match.