LearningRuleTypes have "delta" as first probeable

[hunse]

Motivation and context:

While best practice is to explicitly pass the attr when probing learning rules, it seems sensible to also have some consistent default. The logical choice is "delta", since all learning rules must have this.

Since this is a breaking change (albeit minor), we'll hold off until Nengo 4.0.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Breaking change (fix or feature that causes existing functionality to change)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

Still to do:

• Decide if we want to build this into the base LearningRuleType or LearningRule itself, such that "delta" is inserted at the front automatically.