Finite Difference Derivatives Tutorial

This is work in progress to write some tutorial material for devising finite difference approximations to derivatives. I will end up with at least two major sections:  the first is a direct way of forming finite difference derivatives using SymPy's matrices and linear equation solvers.  While the first method is not the most efficient, I think it is pretty straightforward and serves as a basic introduction.  The second major section will cover the basics of using Bjorn Dahlgren's recently merged finite difference work.  It is quite efficient and I will show how to do the same things as in the first section using finite_diff_weights().

I have  run bin/stripwhitespace on the file finite_diff_derivatives.rst.