Example Suggestions: Heat quation, Lorenz equations, Lotka-Volterra equations and Sturm-Liouville Equation

Hi,

I'm quite the fan of this Python module, and I wanted to suggest the following examples:
- The [Heat equation](https://en.wikipedia.org/wiki/Heat_equation), that is:

- The [Lorenz equations](https://en.wikipedia.org/wiki/Lorenz_system):


Would be best to solve it with the original parameters: ,  and . `x[0] = y[0] = z[0] = 1` on t ∈ [0,100].
- The [Lotka-Volterra equations](https://en.wikipedia.org/wiki/Lotka%E2%80%93Volterra_equations), that is:

- The [Sturm-Liouville Equation](https://en.wikipedia.org/wiki/Sturm%E2%80%93Liouville_theory), specifically the example:


where lambda is the eigenvalue of the equation. This problem is integrated over the range x ∈ [0,∞], although a range of `[0,100]` is good enough to get the first 10 or more eigenvalues. Plus it has the boundary conditions y(0) = y(∞) = 0. This equation has the exact solution:


where Ai is the [Airy Ai function](https://en.wikipedia.org/wiki/Airy_function). The eigenvalues are the zeros of the Airy Ai function, with their negative sign replaced with a positive sign.