Verify notes on discretization of operators

[jlperla]

Check all of the math in the docs/operator_discretization_finite_differences.tex file. This has been separated from the older file, since I realized these notes will apply to all sorts of other examples.

A few things to check carefully:

• I added in the boundary values for both the reflection and the absorbing barrier. Go through the algebra very carefully to make sure I didn't make a mistake in them.
• You will also see that I think the formulas for (7) to (11) simplify in the boundary conditions (especially in the reflection case).
• Do a sanity check that the upwind procedure (i.e. the sign of of $\mu_1$ doesn't effect the reflection boundary values in (17) and (20). My suspicion is that it does not since this seems to make the math add up for each row adding to 1 - as it should as a proper markov chain.
• I think I modified your case of the $\mu < 0$ specialization correctly in (21) to (28), but I would check it out to be sure.

[stevenzhangdx]

Tex file has been checked and minor typos haven been corrected. $A$ has been converted to $\Delta A$ in the discretize_univariate_diffusion function.