Fixes #2643 add snapshot as property for individual and expression building blocks

[rwmcintosh]

Fixes #2643

Description

PR facilitates the exchange of snapshots and core building blocks between MoBi and PK-Sim

Type of change

Please mark relevant options with an x in the brackets.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires documentation changes (link at least one user or developer documentation issue):
• Algorithm update - updates algorithm documentation/questions/answers etc.
• Other (please describe): Task

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Integration tests
• Unit tests
• Manual tests
• No tests required

Reviewer checklist

Mark everything that needs to be checked before merging the PR.

• Check if the code is well documented
• Check if the behavior is what is expected
• Check if the code is well tested
• Check if the code is readable and well formatted
• Additional checks (document below if any)
• Check if documentation update issue(s) are created if the option This change requires a documentation update above is selected

Screenshots (if appropriate):

Questions (if appropriate):

[rwmcintosh]

I also needed a way to serialize building blocks to PKML without writing to file so that they can be shared with MoBi.