Diode support : approximate by unequal forward and reverse bias resistances

A diode is approximated by the linear terms in the Taylor's expansion about some non-zero points (different for forward and reverse bias regimes). The cut-in voltage is assumed to be zero.

For proper implementation, the Z-matrix has to be recomputed and the circuit re-solved at many time instants. This requires modification of code to read the circuit parameters(to display on the oscilloscope et al)

Non-zero cut-in voltage will soon have to be added with a voltage source in series with this diode.