[operators.fv] bugfix
[operators.fv] small tweak
[operators.fv] add NonlinearAdvectionEngquistOsher
[analyticalproblems.advection] add flux_function_derivative option
[analyticalproblems.burgers] add flux derivative
[discretizers.advection] allow to choose numerical flux
[domaindiscretizers.default] bugfix
[demos] extend burgers demo