Category attribute not mandatory in transformation

[KostasPatroumpas]

It is possible that no classification scheme applies to data submitted for transformation. In this case, under a custom profile, the user does not need to specify a file (in .CSV or .YML) representing this classification scheme, and also no category attribute (under the data properties in configuration). Even though the configuration form in the Workbench currently allows this, it still seems to require specification of a category attribute for successfully executing a transformation task, In case that no classification applies and no category attribute is specified by the user, transformation in the Workbench fails. This is not the case when transformation is executed from a terminal under the same configuration.

[drmalex07]

The rpc-server (the server part that schedules workflow execution) regards the classification as a required part.

If the "no-classification" case is not so common (or if it can be hidden under a default classification, supplied by a fallback profile), i suggest that we dont touch this