Solving the Poisson Equation

Recent solver and visualization updates

• [x] Ex_DiffusionSLCN_Cartesian-NL.py

• [x] Ex_DiffusionSLCN_Cartesian.py

• [x] Ex_Poisson_Cartesian_Generic.py

• [x] Ex_Poisson_Cartesian.py

• [x] Ex_Poisson_Gradient_Recovery.py

• [x] Ex_Poisson_Spherical.py