Preliminary experimental work for a voltage-controlled switch.

* It has one threshold `VT` and hyst. `VH`.
* On and off resitances can be set thorugh `RON` (default 1ohm) and
 `ROFF` (default 1/gmin, usually that means 1e12 ohm)
* Transition between the conducting and non-conducting state is
modeled through `tanh(x)`.

It can be extended to a current-controlled switch (CSW in SPICE)
easily.
Re-apply Ian's "kill off trailing whitespace" patch, previously unintentionally reverted.
The module needs MUCH more testing, but for now all results seem OK.
Docstrings, checking and a better test can be run with python -i switch.py
Fix bug, remove verbose printing.
Minimum diode conductance set to gmin/10.
Add parsing a switch and its model from a netlist, unit conversion improvements
Sometimes VON and VOFF are provided instead of VT and VH. Add very preliminary support.
Fix diodes module name
Fix y-axis labels and change markeredgecolor to None.
Fix VON,VOFF to VT,VH conversion and set VH to DeltaV*1e-3
Merge pull request #8 from ahkab/switch

Add a voltage controlled switch