WFQuery refactoring and a new get_nodes_providing() function

1. Corrected indentation in wfquery.py
2. Removed redundant code in get_wf_info()
3. Wrote a function to return a list of fw_ids providing a specific output with
   • string match, regex match;
   • reuse get_fw_info().
4. Now get_data() returns no entries (with data: None) in case when the data is not provided. This is, when inputs provided by parent nodes or outputs are not available due to non-completed computations.