fixed bug in interplay between stepper <-> problem description set

the 0 entry in the problem description set contains the initial guesses
each simulation step works on its own probolem desription beginning
at index 1. the bug became 'active/visible' when several simulations
without stepping are executed, which is the usual case for a gui,
where the user might run severla simualtions with different BC contact values
fixed 'resize_problem_description' for 3d + ohmic contact for n/p

now using ohmic contact electrons/holes model for initial guesses as well,
which is now consistent with the BC