ZBRENT

     Find the zero of a 1-D function up to specified tolerance.

     This routine assumes that the function is known to have a zero.
     Adapted from procedure of the same name in "Numerical Recipes" by
     Press et al. (1992), Section 9.3

       x_zero = ZBRENT( x1, x2, FUNC_NAME="name", MaX_Iter=, Tolerance= )

       x1, x2 = scalars, 2 points which bracket location of function zero,
                                               that is, F(x1) < 0 < F(x2).
       Note: computations are performed with
       same precision (single/double) as the inputs and user supplied function.

       FUNC_NAME = function name (string)
               Calling mechanism should be:  F = func_name( px )
               where:  px = scalar independent variable, input.
                       F = scalar value of function at px,
                           should be same precision (single/double) as input.

       MAX_ITER = maximum allowed number iterations, default=100.
       TOLERANCE = desired accuracy of minimum location, default = 1.e-3.

       Returns the location of zero, with accuracy of specified tolerance.

       Brent's method to find zero of a function by using bracketing,
       bisection, and inverse quadratic interpolation,

       Find the root of the COSINE function between 1. and 2.  radians
        IDL> print, zbrent( 1, 2, FUNC = 'COS')
       and the result will be !PI/2 within the specified tolerance

       Written, Frank Varosi NASA/GSFC 1992.
       FV.1994, mod to check for single/double prec. and set zeps accordingly.
       Converted to IDL V5.0   W. Landsman   September 1997
       Use MACHAR() to define machine precision   W. Landsman September 2002

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