Categorisation functions should make lazy coordinates

[pp-mo]

It looks like functions such as such as iris.coord_categorisation.add_month don't create lazy coords.
Worse, they may actually realise the existing coords which they reference ?

[acchamber]

I Think it does. All the builtins use the _pt_date function, which requires calling the cftime.num2date function on every time point in the coord. each operation is indervidually a few milliseconds, vectorized with numpy, but you can't avoid getting the points realised before using num2date