new contact-related API changes

allows now to set the contacts conveniently from a GUI as well
implemented ohmic contacts for n/p metal-semiconductor contacts

n = 1/2((C^2+4ni^2)^(1/2)+C)
p = 1/2((C^2+4ni^2)^(1/2)-C)

so far, the doping values have been used, also fine but now it's a proper
ohmic contact implementation
revisited 'builtin potential' implementation

the previous one neglected 'polarity', now we use the
plain signed implementation - need to discuss with KR regarding motivation
for the previous one