NAME:
	ZBRENT
 PURPOSE:
	Returns the zero of the function FUNC between X1 and X2 (with accuracy
	TOL)
 CATEGORY:
       BATSEMATH
 CALLING SEQUENCE:
	Z0 = ZBRENT(FUNC,X1,X2,TOL)
 INPUTS:
	FUNC	string; name of the IDL function
	X1,X2	two coordinate values which bound the zero
	TOL	tolerance for the calculated zero
 OUTPUTS:
	ZBRENT	the calculated zero
 CALLED BY:
	FIND_TANGENT, FITEXY, temcal
 RESTRICTIONS:
	The zero must be bracketed by X1 and X2, i.e. FUNC(X1)*FUNC(X2) >= 0.
	The function for which the zero is calculated is called in the form
	F = FUNC(X)
 PROCEDURE:
	See Numerical Recipes, Press and Teukolsky
 MODIFICATION HISTORY:
	APR-1991, Paul Hick (ARC)