Abstracting GridField row class generation

[assertchris]

This further abstracts the classes generated for each row of the GridField. Without it, subclasses need to implement the same first/last/odd/even/base classes, and they also become responsible for the special data-* parameters GridField needs.

[willmorgan]

Hmm, this is now a stealthy API change because of #4079 😉

I think we need to be more careful in the future - we've introduced 2 new methods and then changed their argument signature fairly quickly, and taken them in to 3.1.

Does it matter so much that they're new and protected?

@tractorcow, thoughts?

[assertchris]

@willmorgan "Does it matter so much that they're new and protected?" and there has been no framework release featuring the changes... :)

[tractorcow]

I agree with @willmorgan in that I think it's more a sign we should PR minor changes more carefully, to prevent us having to patch them up like this in the future. ;D I'm as guilty of that as any.

We can probably excuse this one as a replacement PR for the prior.