Add new interface for tagging assistants.

[Issue: #330]
Tag assistants in core/ast.

[Issue: #330]
Merge remote-tracking branch 'origin/development' into ldc/quickies
Rename assistant interface to AstAssistant.

New name is more descriptive.

[Issue: #330]
Add interface marker to POG assistants.

[Issue: #330]
Add interface marker to interpreter def and exp assistants.

[Issue: #330]
Delete hollow assistant.

[Issue: #330]
Tag interpreter pattern assistants.

[Issue: #330]
Tag interpreter stmt assistants.

[Issue: #330]
Delete unused assistant.

[Issue: #330]
Delete vdmj compatibility tests

Not used anymore.

[Issue: close #377]