latch_transfer
==============

Transfer function that implements a "latching" mechanism in one of two variants,
depending on the value of the parameter `keep_value`.

If `keep_value` is zero: All states start at zero.  If the input to a unit is 
negative, set the state to `low_value`; if the input to a unit is positive,
set the state to `high_value`; otherwise, copy the previous state.

If `keep_value` is non-zero: All states start at zero.  If the input to a unit
is non-zero, copy the input to the state; otherwise, copy the previous state.

Parameters
==========

	parameter	default_value
	keep_value	0
	low_value	-1
	high_value	1