Routine Name:	addmsg

Description:	Establishes message links between two elements.

Usage:		addmsg source-element dest-delement msg-type [msg-fields]

Example:	addmsg  /cell/dend /cell/soma      RAXIAL Ra previous_state
		addmsg  /cell/soma /cell/dend      AXIAL  previous_state
		addmsg  /cell/soma /graphs/Vmgraph PLOT   Vm *voltage *red

Notes:		addmsg sets up communication links to pass information
		between elements.  Messages flow along message links created
		by the addmsg routine each time a simulation timestep is
		executed.

		For example, an asymmetric compartment connected to another
		asymmetric compartment needs to send both its axial
		resistance, Ra, and its membrane potential at the previous
		simulation step to the second compartment.  In order to update
		its state, it needs to receive the second compartment's
		previous membrane potential.

See also:	deletemsg, showmsg, getmsg, gen3dmsg, dd3dmsg

