[PPSS] Add setters & input validation (merge from PR #1281)

[trentmc]

Background / motivation

PR #1281 is a huge PR towards issue #1281. Some of it I may never merge. But some of it is immediately good to have in main, to minimize the PR size.

I have many good changes to pdr_backend/ppss/ that could move over. And related unit tests.

• validating inputs
• setters
• expanded unit tests

TODO

• Move ppss changes to main