Laplace Eq. test and docu

...iptic/square_1x1_GroundWaterFlow_Python/bcs_laplace_eq.py

...uare_1x1_GroundWaterFlow_Python/python_laplace_eq_ref.vtu

...uare_1x1_GroundWaterFlow_Python/square_1e3_laplace_eq.prj

...Elliptic/square_1x1_GroundWaterFlow_Python/square_1x1.gml

...square_1x1_GroundWaterFlow_Python/square_1x1_quad_1e3.vtu

...enchmarks/python-bc/laplace-equation/python-laplace-eq.md

...rks/python-bc/laplace-equation/python_laplace_eq_diff.png

...python-bc/laplace-equation/python_laplace_eq_solution.png