Update PackageVerificationCode to have algorithm explicit

[kestewart]

In #643 the PackageVerificationCode model had the algorithm, but the text for the class had it hardcoded as SHA1. The model was updated to match, but the algorithm should be put back into the diagram & the description of packageVerificationCode rewritten to include use of algorithms other than SHA1.

[kestewart]

Need to get example to see how this is able to express with different HASH algorithms - I think we need to add in the "algorithm field" to the class. Thoughts?

[kestewart]

DIscussion from the call, preference is to replicate algoirthm in PackageVerificaitonCode.

PR will be submitted.

[goneall]

Resolved with PR #699