The structure of the driver is copied from the Vhdl one. It is not
integrated as a an AllianceFramework one but as a standalone like
GDS. For now use numerical indexes for electrical nodes but also
support strings. The nets are ordereds in reverse alphabetical
order, but a custom order can be defined, if we read the model
from an external SPICE subckt (to be done).
SPICE saving has also been added to the cumulus/rsave plugin
and the block/chip P&R one.