Make KernelCenterer a _pairwise operation
Make KernelCenterer a _pairwise operation

Replicate solution to https://github.com/scikit-learn/scikit-learn/commit/9a520779c233dfeff466870c0b7cb04b705e61af except that `_pairwise` should always be `True` for `KernelCenterer` because it's supposed to receive a Gram matrix. This should make `KernelCenterer` usable in `Pipeline`s.

Happy to add tests, just tell me what should be covered.
imaculate/scikit-learn,imaculate/scikit-learn,imaculate/scikit-learn,imaculate/scikit-learn