Add way to create new version that changes the digest algorithm

[zimeon]

In order to be able to write up an implementation note describing how the creation of a new version may be used to update the digestAlgorithm (OCFL/spec#430) it would be helpful to have code that implements it...

[zimeon]

Useful test sequence:

rm -rf tmp/spec-ex-min-change-hash

./ocfl-object.py --create --srcdir fixtures/1.0/content/spec-ex-minimal/v1 --objdir tmp/spec-ex-min-change-hash --id ark:arf/change/me/1 --digest sha256 --created 2020-03-26T20:33:00 --message "sha256 for EVER"  --name "Secret Agent" --address secret@example.org

./ocfl-object.py --show --objdir tmp/spec-ex-min-change-hash

./ocfl-object.py --validate --objdir tmp/spec-ex-min-change-hash

./ocfl-object.py --update --objdir tmp/spec-ex-min-change-hash --digest sha512 --created 2020-03-26T21:00:00 --message "Update sha256 to sha512, no content change" --name "Special Agent" --address "special@example.org"

./ocfl-object.py --show --objdir tmp/spec-ex-min-change-hash

./ocfl-object.py --validate --objdir tmp/spec-ex-min-change-hash

[zimeon]

Used to create examples for implementation notes:

rm -rf tmp/spec-ex-min-change-hash
./ocfl-object.py --create --srcdir fixtures/1.0/content/spec-ex-minimal/v1 --objdir tmp/spec-ex-min-change-hash --id http://example.org/digest_update_example --digest sha256-spec-ex --created 2020-01-01T01:01:01 --message "sha256 forever"  --name "Secret Agent" --address secret@example.org
./ocfl-validate.py --lax-digests --no-check-digests tmp/spec-ex-min-change-hash
./ocfl-object.py --update --objdir tmp/spec-ex-min-change-hash --digest sha512-spec-ex --created 2020-03-26T21:00:00 --message "Update sha256 to sha512, no content change" --name "Special Agent" --address "special@example.org" --lax-digests

[zimeon]

Working fine and example created by this method was merged into the implementation notes (#12)