Inverse (integrated) gradient

Vectorized & fully sparse 1-d, 2-d, & 3-d integrated gradients

The inverse of the gradient function. I've provided versions that work on 1-d vectors, or 2-d or 3-d arrays. In the 1-d case I offer 5 different methods, from cumtrapz, and an integrated cubic spline, plus several finite difference methods.

In higher dimensions, only a finite difference/linear algebra solution is provided, but it is fully vectorized and fully sparse in its approach. In 2-d and 3-d, if the gradients are inconsistent, then a least squares solution is generated.

(I'll enhance the 2-d and 3d tools if there is any interest. Currently they are set to be 2nd order methods on uniform grids.)

Please notify me of any bugs.