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
• 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?