2D Vorticity with any stencil

Computes curl() with arbitrary number of points in the stencil

curl(X,Y,U,V) in Matlab will give you the angular velocity or vorticity of a 2D vector field (U,V) with a 3-point stencil. This implementation will compute the curl of a **UNIFORM*** grid with a 'npts' point stencil (only supports central differences). Since it uses convolution to accomplish that, the corners and edges of the grid will be incorrect.

Using for PIV post-processing, where noise can be an issue when computing derivatives.

Shoutout to Greg von Winckel for providing the stencil generator code (https://www.mathworks.com/matlabcentral/fileexchange/5269-finite-difference-weights)!