MOSFET output resistance

[simgug]

Add the possibility to model the MOSFET output resistance, even the first order channel length modulation would be enough (R0 = ID / VA).
Otherwise, give the possibility of using SPICE-like models like for the BJTs

[esaruoho]

PR #240 adds a channel-length modulation parameter (lambda) to MOSFETs and JFETs. In saturation, Ids = 0.5beta(Vgs-Vt)^2*(1+lambda*Vds), giving finite output resistance. The parameter defaults to 0 (ideal square-law, backward compatible) and appears as a new field in the element edit dialog.