decoupled majority of *.hpp files into headers/sources
fixed PDE sets: laplace and DD::both PDE sets work now

added 'contacts' to the 'permittivity' roles -> this distributes the
permittivity also on contact segments, which is required by ViennaFVM.
simplified device::make method + reactivated mosfet 2d/3d devices

mosfets currently don't work, as their is a bug in the implementation of the ohmic contact models
with regard to handling oxide segments correctly.