Eradicate last traces of GPL licensing

examples/advection/weak.py

unported-examples/advection/advection.py

unported-examples/burgers/burgers.py

unported-examples/gas_dynamics/box-in-box.py

unported-examples/gas_dynamics/euler/sine-wave.py

unported-examples/gas_dynamics/euler/vortex-adaptive-grid.py

unported-examples/gas_dynamics/euler/vortex-sources.py

unported-examples/gas_dynamics/euler/vortex.py

unported-examples/gas_dynamics/gas_dynamics_initials.py

unported-examples/gas_dynamics/lbm-simple.py

unported-examples/gas_dynamics/naca.py

unported-examples/gas_dynamics/navierstokes/shearflow.py

unported-examples/gas_dynamics/square.py

unported-examples/gas_dynamics/wing.py

unported-examples/heat/heat.py

unported-examples/maxwell/dipole.py

unported-examples/maxwell/inhom-cavity.py

unported-examples/maxwell/maxwell-2d.py

unported-examples/poisson/helmholtz.py

unported-examples/poisson/poisson.py