laplace/poissonDD now use the previous simulation results as initial guess

up untiil now, the initial conditions for the first iteration were
used for all simualtions in a BC range, obviously not optimal.

however, also this approach requires future revisions, if there are more
than two contacts and two ranges are processed, in this case the initial
guesses based on the 'previous' solution might not be optimal
reactivated lattice/ionizied impurity scattering mobility models