reordered args in new cube.interpolate() api

`cube.regrid` and `cube.interpolate`, both take a target grid/sample_points and a scheme, however, the order of those objects in the signature is not consistent. This PR changes `cube.interpolate` to match that of `cube.regrid` i.e.

`cube.interpolate(sample_points, Linear())`

matching:

`cube.regrid(grid_cube, Linear())`