As discussed in #45 we want to avoid implicit numerical differentiation as this might confuse users. * We want to update the PDELab internal local operators so that they don't use numerical differentiation. * In the future we consider numerical diff in an official local operator a bug! * Currently we have to fix the following operators: * [ ] convectiondiffusionccfv.hh * [ ] convectiondiffusiondg.hh * [ ] convectiondiffusionfem.hh * [ ] diffusionmixed.hh * [ ] electrodynamic.hh * [ ] linearacousticsdg.hh * [ ] maxwelldg.hh * [ ] nonlinearconvectiondiffusionfem.hh * [ ] twophaseccfv.hh