Porous flow examples

e.g. groundwater equations / Darcy flow

Recent solver and visualization updates

• [ ] Ex_Darcy_Cartesian.py

• [ ] Ex_Darcy_1D_benchmark-Swarm.py

• [ ] Ex_Darcy_1D_benchmark.py

• [ ] Ex_viscousFingering.py

• [ ] Ex_viscousFingering-Viz.py