Script for the Laplace equation
Added a script that solves and graphs the Laplace equation in 2D.
The setup of both variables and matrices of the discretization
is in a different script for convenience.
Script for the heat equation
Added a script that solves and graphs the heat equation in 2D, using
Crank-Nicolson discretization, and BICGSTAB to solve the linear system.

The setup for the Laplace equation has been reused.