Ocfl-py

* Validator now checks fixity block structure, additional fixity values in fixity block
* Validator now checks for repeated digests in manifest, fixity and state blocks (https://ocfl.io/1.0/spec/#E096, https://ocfl.io/1.0/spec/#E097, https://ocfl.io/1.0/spec/#E098)
* Move all the many README_*.md demos into docs folder
* Add build_demo_docs.sh to build demo descriptions in docs folder

* ocfl-validate.py script now handles storage roots and objects
* When validating, show warnings and errors by default (https://github.com/zimeon/ocfl-py/issues/22)
* Handle case of no versions https://ocfl.io/1.0/spec/#E008
* Handle case of bad JSON inventory https://ocfl.io/1.0/spec/#E033
* Handle case of no manifest https://ocfl.io/1.0/spec/#E041
* Handle case of conflicting paths https://ocfl.io/1.0/spec/#E095

* Renumber errors to align somewhat with the canonical code set extracted
at https://github.com/OCFL/spec/blob/main/validation/validation-codes.md
* Add --version parameter to scripts to show version number

* Actually check digests for content during validation!
* Add --validate to ocfl-store.py to check all of storage root

* Add missing package data to install
* Deal with tests for warnings https://ocfl.io/1.0/spec/#W003,
https://ocfl.io/1.0/spec/#W009 and https://ocfl.io/1.0/spec/#W010

* Two in a day...
* Support for version creation from and extraction as Bagit bags