Rename AdditiveClosure into StrictAdditiveClosure

[mohamed-barakat]

I suggest renaming AdditiveClosure into StrictAdditiveClosure since its universal property only works for functors into strict additive categories (those for which the unitors and associators of the direct sums are identities).

[TKuh]

@mohamed-barakat Are you still in favour of renaming?

[mohamed-barakat]

Yes.

[TKuh]

Ok, then I suggest that I do it. I am working on the AdditiveClosure's anyway.