Improve theusarus handling

[etj]

When loading a thesaurus, the logic should expect that it already exists. In this case:

• new keywords should be added.
• new localizations for an exisiting keyword should be added
• changed localizations shold be updated
• deleted keywords should be deleted IF they are not references in a dataset.

[etj]

Command entrypoint is https://github.com/geosolutions-it/ckanext-dcatapit/blob/master/ckanext/dcatapit/commands/dcatapit.py