Import operation with not existing AlgorithmGroup

When a user import a project which contains operations that refer an AlgorithmGroup that doesn't exists anymore into the application then all the datatypes resulted from those operations wont't be visible into the ui.

If the user tries to delete the imported project then the datatypes resulted from those operations won't be deleted.

Possible solution:
When we encounter a such operation we could set its algorithm group to tvb_importer.
The change could be done into __build_operation_from_file method from importservice or into from_dict method from model_operation.