The ddsyn channel is similar to a synchan with an internal table to transform
voltage to activation.  In a typical usage, it receives its activation, not
in the form of a delta-function spike from an axonal connection, but from
its internal table in repsponse to a voltage sent from the pre-synaptic
dendrite compartment.  This allows it to model dendro-dendritic synapses.

The ddsyn.g script creates a ddsyn with a V_PRESYN message
and an equivalent combination of a synchan getting an ACTIVATION message
from a table.  Various values of voltage are sent to ddsyn or the external
table as the simulation is stepped in increments of 100 msec.
