Add n-dimensional m-derivative generalized finite difference

[AndreyAPopov]

The PDE utilities we have right now are lacking, and something more general is required.

I am currently working on supported GFDM in OTP, which hopefully should be able to support a significantly larger class of problems than the current very limited tools that we have right now.

AAP

[Steven-Roberts]

Let's discuss at next meeting. We need to plan out the desired features.