Padding

[nilgoyette]

This PR modifies the pad function to

• accept n-dimensional arrays (instead of only 3D)
• Add almost all NumPy pad modes. Only linear_ramp and <function> are missing. I do not expect to add them now or ever.
• Add several tests for all modes

I believe it is much cleaner than NumPy version but it is also slower :( In fact, we're around 2-4x slower than NumPy, depending on the mode. See #4 for more information.

The ndarray-stats dependency was added.

[nilgoyette]

Also as a general comment I think we could use https://docs.rs/ndarray/latest/ndarray/struct.ArrayBase.html#method.multi_slice_mut To try to simplify the data manipulation no ?

Using multi_slice_mut could replace my 2 buffers, but I don't believe it's possible:

• One is used with Median. I can't avoid it because I must sort the data.
• The other is used in ByIndices. It looks like it could be replaced but I'm sure it can't. We're only writing in the left and right part, but we're reading the whole lane. multi_slice_mut must be able to prove that both slices don't touch. They do touch.