Sort and order

[mpsonntag]

This PR mainly introduces a SmartList.sort method and adds specific comparison magic methods to the SmartList.

• Adds SmartList.sort(). By default Document and Section child lists will retain the order in which child elements were added, but now a sort by name can be manually triggered.
• Adds SmartList comparison magic methods to partially address Comparison of odml objects #265. The introduction of the RDF backend led to an issue when comparing odML entities. The used RDF library rdflib does not respect child order upon loading of a file, odML entities with children can not be compared without sorting the child elements. The added magic methods sort child elements by name before comparison without changing the actual order of the child elements. This only addresses the issue for Section and Property child lists, but does not solve the problem for the order of Property.values.
• Adds tests for all levels of entity comparison.
• Adds a changelog to the project.
• Updates the sphinx documentation config file to the latest odml version.

[achilleas-k]

LGTM