Move sliding_window_view to the main module and add step_size

Writing np.lib.stride_tricks.sliding_window_view is a bit of a mouthful, I think it would be better if it was placed inside the main module. (also doing import from is a bit inconvenient)
I suggest adding a step_size parameter to the function, an example of its use would be:

x = np.arange(10)
sliding_window_view(x, 3, step_size=2) 

>> array([[0, 1, 2],
          [2, 3, 4],
          [4, 5, 6],
          [6, 7, 8]])

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