CIRCUM - define CIRCUMcircles for Delaunay triangulation

circum - compute center and radius-squared of circumcircle of 3 (x,y)
          locations
circumTri - compute center and radius-squared of circumcircle of 
            triangular face

Function Prototypes:
void circum (float x1, float y1, float x2, float y2, float x3, float y3,
	float *xc, float *yc, float *rs);
void circumTri (Tri *t);
circum:
Input:
x1	x-coordinate of first point
y1	y-coordinate of first point
x2	x-coordinate of second point
y2	y-coordinate of second point
x3	x-coordinate of third point
y3	y-coordinate of third point

Output:
xc	pointer to x-coordinate of center of circumcircle
yc	pointer to y-coordinate of center of circumcircle
rs	pointer radius^2 of circumcircle

circumTri:
Input:
t	Pointer to Tri	

Returns:
xc	x-coordinate of circumcircle
yc      y-coordinate of circumcircle
rs      radius^2 of circumcircle

Author:  Dave Hale, Colorado School of Mines, Fall 1990.