Higher order gradient calculation

[MaceKuailv]

This is part of the JOSS paper review openjournals/joss-reviews#7873

np.gradient is used for most of the gradient calculation, which is a second order gradient operator. My worry is that it may not be enough for higher order gradients. It would be great if you can implement some higher order gradient functions. At least, the users should get a warning when doing something like

sf = fluidsf.generate_structure_functions_2d(U, V, x, y, sf_type=["LLLLLLLLL"], boundary=None)