solved the issue wf_query, fw_ids and wf_ids as properties

In the WFEngine class there are many instances of this workflow query: {'nodes': {'$in': self.wf_ids}}. These can be replaced by a wf_query that in turn should be a property method. Also wf_ids should become a property method that automatically updates the wf_query. The other way around, the wf_query property method should update the wf_ids property. Also the get_fw_ids() should be replaced by a property method fw_ids.

Closes #37 (closed)