Update PDELab local operators to not use numerical differentiation

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