Range helpers, new skeleton, various fixes

[digitaldogsbody]

Allows Ranges to be added by reference + adds make_range helper
Allows other item types to have thumbnails (NB: this will make some tests fail until IIIF/presentation-validator#151 is merged, and the skeleton.py file regenerated)

closes #45

[glenrobson]

OK I've updated the schema so you should be able to update skeleton.py

[digitaldogsbody]

Skeleton py is updated - and various fixes have been made and also included in this PR

• Add an Auto for items to ensure it always exists when required (e.g was causing validation errors when trying to instantiate a canvas)
• Fixes to pydantic setup to try and stop unexpected coercions when the object type is permitted
• Incorporated the new reference classes into code and tests
• Fixed / cleaned up some old tests