`xt::pad`: more algorithms & recursion

[tdegeus]

To make xt::pad more general and more compliant with NumPy, the xtensor implementation should be extended with:

• More (all) algorithms that NumPy features.
• Recursively call of pad if the pad_width along an axis >= e.shape()[axis] (only for algorithms: periodic, symmetric, [extend with future additions]).

[tdegeus]

See implementation of xt::tile to make xt::pad recursive (where possible).