Bring cubelist "extract_strict" into line with other Iris methods

[pp-mo]

There's a tweak to CubeList.extract_strict which makes it return a single cube as it is, instead of return a "list of one cube".
We just documented this more fully
But, as mentioned there I don't personally like returning different types of object from different operations, which I think is a bit of an anti-pattern...

IIRC this is why we have provided other functions in multiple convenience versions,
like "load or load_cube" and "cube.coords or cube.coord" : these forms combine convenience + syntactic elegance with better clarity as to the exact function + interface.
Even more relevant, right here in CubeList, we have merge() / merge_cube() and concatenate / concatenate_cube.

In this case, there is a bit of an obstacle in the dual choice between "cube or cubes", and "strict or not". I guess we are not going to have all of "extract, extract_strict, extract_cube, extract_cube_strict" ...

To be definite, I propose we should drop extract_strict altogether,
and replace it with extract_cube and extract_cubes.
I think that works, and would be much more in line with the other methods.

So, for examples of usage change :

• extract_strict(some_constraints) --> extract_cubes(some_constraints).
• extract_strict(one_constraint) --> extract_cube(one_constraint).

[rcomer]

extract_cubes would also be consistent with load_cubes 👍

[pp-mo]

#3715 addresses