Specific assocciations should be handled as compositions in QGIS

[signedav]

In INTERLIS you can define three types of strengths in an ASSOCIATION:

• Association .. : objects exist independently
• Aggregation .<> : Child objects belong to one (or no) parent. They should be copied with the parent, but not deleted.
• Composition .<#>: Child objects belong to a parent. They should be copied with the parent and also deleted.

Normally the QGIS relations of such compositions also become compositions in QGIS and associations become associations. Aggregations - i.e. the use case with copying but not deleting - are not supported in QGIS, which is why they also become associations.

However, new associations or aggregations sometimes also become compositions in QGIS.

1. if the cardinality is exactly 1:

    ASSOCIATION fakecomp1 =
      fakecomp_1a -- {1} ClassA1;
      fakecomp_1b -- {0..*} ClassB1;
    END fakecomp1;

or

    ASSOCIATION fakecomp2 =
      fakecomp_2a -<> {1} ClassA1;
      fakecomp_2b -- {0..*} ClassB1;
    END fakecomp2;

Because such an object cannot exist without a parent. This fixes

2. if it is a linking table:

    ASSOCIATION assoc =
      assoc_a -- {0..*} ClassA1;
      assoc_b -- {0..*} ClassB1;
    END assoc;

Which results in two QGIS relations (normalized) with a linking table.

If one of the parents is deleted, the linking table entry is also deleted (but not the other linked parent).

[gacarrillor]

I don't have too much to say here :)

Looks solid with the unit tests!