Function:     spline1

Purpose:      1D cubic spline interpolation.

Category:     MATH

File:         spline.c

Author:       P.R. Roelfsema

Use:          INTEGER SPLINE1( XI   ,               Input   real( > NIN )
                               YI   ,               Input   real( > NIN )
                               NIN  ,               Input   integer
                               XO   ,               Input   real( > NOUT )
                               YI   ,               Output  real( > NOUT )
                               NOUT ,               Output  integer

              SPLINE1 returns error codes:
                      >0 - number of undefined values in YI.
                       0 - no problems.
                      -1 - not enough memory to create internal tables.
                      -2 - the input array has two equal x-coordinates or
                           value in XO outside range of input coordinates >
                           interpolation problems.
              XI      Array containing input x-coordinates.
              YI      Array containing input y-coordinates.
              NIN     Number of (XI,YI) coordinate pairs.
              XO      Array containing x-coordinates for which y-coordinates
                      are to be interpolated.
              YO      Array containing interpolated y-coordinates.
                      If ALL YI are undefined, ALL YO are set to undefined.
              NOUT    Number of x-coordinates for which interpolation
                      is wanted.

Description:

		The interpolation is based on the cubic spline interpolation
	routines described in "Numerical recipes in C". For the interpolation
	data points in YI wich are undefined are skipped. Thus if undefined
	values are present, this routine will interpolate across them. 
	XI data must be in increasing order. The number of undefined values 
	in YI is returned.

Updates:      Jan 29, 1991: PRR, Creation date
              Aug  7, 1991: MV,  Reset of 'klo' in 'spline1_c'
                                 Documentation about XI.
              Aug  9, 1993: PRR, Set YO to undefined when all YI undefined.