[WIP] Learning rate refactor

[rijobro]

Addresses #1454 (review).

@wyli is this what you had in mind by abstracting out the loss functionality? Whilst this works (and is programmatically cleaner), it's s shame that what was previously achieved in a few lines of code for the end user is now more like 150 (see the extra code added to the test script).

Status

Work in progress

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh --codeformat --coverage.
• Quick tests passed locally by running ./runtests.sh --quick.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.