Switch the way definitions store association info.
Rename attach/detachAttribute on model entities.

... to match associate/disassociate terminology
in smtk::attribute.
Implement checks that allow/disallow association.
Get tests working with newly-implemented checks.

The new checks on whether an association is allowed
or not broke a couple tests. Fix things.
Process XML associations as ModelEntityItemDefinitions.

The reader should continue to accept old-style associations
but both now accept/generate child elements of AttDef named
AssociationsDef that are defined equivalently to
ModelEntityItemDefinition instances.
Merge topic 'model-associations'

7a4621a Process XML associations as ModelEntityItemDefinitions.
fc3038f Get tests working with newly-implemented checks.
0f351af Implement checks that allow/disallow association.
00ff040 Rename attach/detachAttribute on model entities.
13cfabc Switch the way definitions store association info.
Fix brown-paper-bag error.
Merge topic 'model-associations'

2bfde2b Fix brown-paper-bag error.
Finalize use of ModelEntityItem to store associations.

Previous changes stored the association membership mask
in a ModelEntityItemDefinition. This commit stores the
actual array of associated entities in an item derived
from that definition.

This also consolidates membership testing between
attribute associations and model-entity attribute items.

Finally some membership tests are fixed.
For example, groups should be allowed to associate to
an attribute missing the "group" bit from its membership mask
(as long as it is also missing the "no group" bit).
Merge topic 'model-associations'

11977dc Finalize use of ModelEntityItem to store associations.