Automatically add missing requirements to decorators

[rg936672]

What's the new feature?

BinaryClassification and CategoricalClassification both require other decorators to be applied before them, and will throw a TypeError if this is not the case. It would be more ergonomic for users if this was done automatically if the decorators aren't already provided.

What value does this add?

Ease of use for client code.

Is there an alternative you've considered?

Leaving it as is.

Additional context

Came up in #379 - that PR introduces a similar mechanism but only within test_decorator_combinations.

We should also add a way of disabling this automatic addition of required decorators, and also a way of disabling the requirement checks, since clients may want to implement their own versions of the required decorators instead. These disables should be signposted with appropriate warnings, though!