fix associations in logical data model
Improved error messages
Ticket 437
Merge pull request #8 from sjcjoosten/master

Improved error messages
Move getEditableRelation to ampersand

We could also choose to store the result of this function in the interface object data structure (ie. ObjectDef), but since this is an A-datatype rather than an Fspec datatype, this would require quite some refactoring to prevent import cycles. It is also debatable whether this kind of information should be part of the A-structure (storing it only in the Fspec is not doable as we would need to copy the tree structure or do some nasty kind of lookup).