Easy build finite-difference operators

Using Fornberg's weights algorithm [Fornberg, Bengt. "Generation of finite difference formulas on arbitrarily spaced grids." Mathematics of computation 51.184 (1988): 699-706.], this example shows how to construct its associated 2D/3D operators for producing differentiations on 2D/3D discrete functions.

This example seeks to be a complement to my 'Easy Build compact schemes' contribution. As this strategy can be directly applied to compact schemes too. ; )

This contribution contains three examples:
* An example on the use of Fornberg's weights algorithm to create FD schemes such as: centered, biased schemes over regularly- or irregularly-spaced domains.
* An example to construct a 2D differential and partial differential operator for a given function.
* Lastly, an example to construct a 3D differential and partial differential operator for a given function.

Happy coding !