Spectral

My edits to the lab on the pseudospectral method for boundary value problems.

- [ ] Finish removal of discussion of Runge's phenomenon.
- [ ] Introduce the method using Neumann boundaries instead of Dirichlet. Develop it as a system of linear constraints.
- [ ] Move Chebyshev differentiation and integration to the Chebyshev lab and introduce how to do it with the fft.