Support real diodes and real resistors.

[hneemann]

So far, no resistors are available. This makes it impossible to simulate some CMOS circuits (See #22, #29)

In CMOS circuits short circuits often occur for very short time. During this time a large voltage drop occurs at the conductive FET and a large current flows. However, different voltage levels may exist on both sides of the FET because of its residual resistance. After a short time, one of the short-circuiting transistors opens and the current disappears. This produces a sharp current peak at the moment of switching. In digital, a conducting transistor has no resistance. Source and drain are shorted out in the conducting state and therefore can not have different voltage levels. This causes a signal change to have no effect. Source and drain have always the same voltage level. The transistor which would open due to the different voltages remains conductive. In this case, an error message appears and the simulation is terminated.

Therefore, the simulation of some FET circuits does not work properly. To solve this problem, it must be allowed for a transistor to have different voltages at the source and drain even in the conducting state. This means that the residual resistance must not be zero.

This problem is related to the implementation of real diodes.

To overcome the issues caused by the missing residual resistance, the FET has an option to make it unidirectional. This is an ugly hack, but it helps to make more circuits work.