Removed the lazy keyword to collapsed.

Removes the "lazy" keyword in ``collapsed`` and deals with any fallout.

(May need to add the deprecation to some documentation somewhere)
I can see the desire to just use numpy here, but I like a number in this simple case. I'm keen we check Iris is doing the right thing, not necessarily that it does what numpy does, which in the case of masked array handling is not as consistent as I'd like. Does this need to be in this PR?