Bug: fix sampling when not given

[AmirMardan]

For trace interpolation, it is necessary to define either sampling or both taxis and spataxis. In case of using the latter, the function SeismicInterpolation is supposed to calculate the sampling. However, the calculated sampling was always [0, 0].

This PR fixes this problem.

[AmirMardan]

Please note that this modification is applied only for 2D problems. It seems, the same modification should be applied to 3D, here

[mrava87]

@AmirMardan oh yeah, good spot 😄 it should have been

 sampling = (
                        np.abs(spataxis[1] - spataxis[0]),
                        np.abs(taxis[1] - taxis[0]),
                    )

but since these are pre-computed above, your fix is actually better. Do you mind push the same change for the 3D case before I merge the PR?

[AmirMardan]

@mrava87 It's done