session: docs, PEP8 and PEP257 improvements

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
base: fix database create error message

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
workflows: cleaning of engine calls

* Removes generation of another object snapshot when continuing a
  workflow causing many duplications in the database.

* Reuses the same workflow when continuing a workflow, as it was
  originally intended.

* Improves current workflow integration to be cleaner, using defined
  interfaces instead of updating closed variables.

* Adds new restart point, 'first', to re-start a workflow engine from
  first object and/or first task (callback).

* Removes resetting of engine `extra_data` when restarting workflow.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibClassify: case insensitive taxonomy

* Fixes a bug where the look-up of ontology names from the
  taxonomies were forced to lower case.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibClassify: PEP8 and PEP257 fix

* Improves PEP8 and PEP257 across ontology_reader.

* NOTE Removes any rdflib checks which are no longer needed as this
  library is (1) mandatory and (2) mandatory at a high version.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibClassify: error handling and PEP8

* Adds a new exception type TaxonomyError when there is a problem
  reading the ontology passed to legacy BibClassify.

* Performs some PEP8 fixes across ontology_reader.py and engine.py.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>