Force Party instantiation to be made from Families

[calum-chamberlain]

What does this PR do?

Raises an assertion error if the wrong type is provided to Party on init

Why was it initiated? Any relevant Issues?

I frequently accidentally instantiate a party with a string. This should not be allowed.

PR Checklist

~~- [ ] develop base branch selected?~~

• [x] This PR is not directly related to an existing issue (which has no PR yet).
• [ ] All tests still pass.
• [ ] Any new features or fixed regressions are be covered via new tests.
• [ ] Any new or changed features have are fully documented.
• [ ] Significant changes have been added to CHANGES.md.
• [ ] First time contributors have added your name to CONTRIBUTORS.md.