@Override
	public String toString() {
		return direction + " - row " + Integer.toString(getRelativeRow());
	}
Returns the child level of this level.  The child of a level has the same direction 
as this level, with a distance of one more than this level.

@param the direction of the child
@return returns the child level of this level
@throws IllegalArgumentException if this is the source level, which is row 1
Ghidra/Features/GraphFunctionCalls/src/main/java/functioncalls/graph/FcgLevel.java