Adding lab on the wave equation
Add draft of second finite difference lab for the 2d heat equation and Poisson's equation
Merge branch 'WaveEqn' into develop

Adding lab on the wave equation
Merge branch 'poisson' into develop

Adding lab on finite difference method for 2d problems
Merge pull request #557 from joshualy/develop

Added labs on the wave equation, and finite difference methods for 2d heat equation, poisson's equation