Matrix

Matrix is one of Pablo's display functional units.  It accepts a two-
dimensional array of input values and represents the data as individual 
squares in a grid pattern accessible by row/column indexing.  Each square
is filled according to the magnitude of a datum and the "minimum" and 
"maximum" data values stored within the functional unit.  The minimum and
maximum values may be set by the user during the configuration process.

The squares within a matrix are filled with a solid color:  the color varies
with the magnitude of the data according to the colormap defined in the
functional unit.

The top-left square of the matrix is defined as row 0, column 0.  Increasing
rows are to the right and increasing columns progress downward.

Clicking the left mouse button with the cursor inside the matrix invokes a
callback displaying the current value at that (row,column) location and
also the row and column location of the click.

Functional Unit Inputs:  array

Matrix attributes:

	XtNheight	height (in pixels) of the matrix widget
	XtNwidth	width (in pixels) of the matrix widget
	XtNbackground	the background color of the widget
	XtNgridColor	color of the matrix horizontal and vertical grid lines
	XtNnumColors	number of different colors in the color scale used
				to fill squares of the matrix
	XtNrows		number of row elements in the matrix
	XtNcolumns	number of column elements in the matrix
	XtNshowGrid	set to TRUE if the matrix grid lines are to be drawn
	XtNminValue	minimum data value represented by the widget
	XtNmaxValue	maximum data value represented by the widget
