Cleanup of comparing to max level in refinement check

[pgrete]

          I see. That makes sense.

I'm still not sure this check should belong inside the kernel (rather than being a general feature of those refinement criteria derived from what's available within the refinement package).
Conceptually, I see the refinement critera itself as what's inside the kernel and the max level an outer constraint (though also being available on a per refinement criteria level).
So I see a danger if there are more refinement criteria are added the code for checking the max level would need to be duplicated and added to each kernel, rather than checking separately.

Having said that, I'm happy to fix this later so that this PR can go in quicker.

Originally posted by @pgrete in #1182 (comment)