#21 Limit Ron for the standard SPICE switch

[marcel hendrix]

In XSPICE the on resistance of the aswitch device is limited between 1mOhm
and 1/gmin. I found it useful (for correct convergence) to do the same
for the standard SPICE sw device.

Unfortunately, it proved too difficult for me to access the internal
1/gmin value from a non-xspice file. However, I don't think that matters
much (file swmparam.c appended).

-marcel