new 'pn 2d diode' template+fixed/improved stepper+minimized poissonDD

@pn 2d diode: the template generates a four segment device:
   anode, P, N, Kathode
   also the template assigns a default doping
  -> corresponding example shows the utilization
  --> plan is to investigate I/V characteristics functionality of ViennaMini

@stepper: fixed range computation for ranges containing negative values
   also fixed output in 'write()' method for improved readibility

@ poisson DD problem: for now (dbeugging) reduced implementation to
  basic poisson-DD problem (deactived additional models)

-> first need to ensure that the solving distributions are correct