fix: Correct offset in gridtools2d, use `compute_derivative`

[jrwrigh]

• Newer versions of pyvista check that size(offset) == size(cellArray), which was off by one (but had no consequences actual consequences)

• Replaced the compute_gradient method with compute_derivative that I contributed (https://github.com/pyvista/pyvista/pull/837)

  • Also replaces (and thus deprecates) vpt.compute_vorticity