Saveload custom func

[newville]

This fixes some of the save/load issues lingering after #932, and fixes some test deprecatiosn

Here we add a "version 2" of the JSON encoding of ModelResult, now relying on Parameter.dumps(). Tests are not altered, and should still all pass.

Type of Changes

• [ ] Bug fix
• [ ] New feature
• [x] Refactoring / maintenance
• [ ] Documentation / examples

Tested on

Verification

Have you

• [ ] included docstrings that follow PEP 257?
• [ ] referenced existing Issue and/or provided relevant link to mailing list?
• [ ] verified that existing tests pass locally?
• [ ] verified that the documentation builds locally?
• [ ] squashed/minimized your commits and written descriptive commit messages?
• [ ] added or updated existing tests to cover the changes?
• [ ] updated the documentation and/or added an entry to the release notes (doc/whatsnew.rst)?
• [ ] added an example?