Importing rdfs schemas

[francescalb]

Description

Partially closes #811.
Closes #797

I is not possible to easily access classes of typ rdfs:Class. This is needed if working simultaneously with an rdf-schema (as opposed to an owl ontology). Owlready2 does not search for enties that are not of type owl:XXX.
A search for rdfs:Class is now added so that when asking for classes in an ontology also the rdfs:Class'es are returned as Python objects.

This is not as readily fixed with rdfs:Property. As a first fix, such properties are now returned as a list of strings (instead of a list of python objcets). Note that the nodes are present in the onotology, but the rdfs:Poerty is not cast as a python object.

Owlready2 is designed to support owl ontologies (not rdfs). If we really want to extend the suport for rdfs:Property the best solution is most likely to cast it to an owl:Annotation_property. If so this should be done in a next PR .

[codecov]

Codecov Report

Attention: Patch coverage is 80.76923% with 5 lines in your changes missing coverage. Please review.

Project coverage is 72.99%. Comparing base (de542a3) to head (401f1d7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ontopy/ontology.py	80.76%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
+ Coverage   72.90%   72.99%   +0.09%     
==========================================
  Files          18       18              
  Lines        3790     3814      +24     
==========================================
+ Hits         2763     2784      +21     
- Misses       1027     1030       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.