CALL INTDER(X,FX,NPTS,X0,FX0,FPX0,IORDER,I) -- This subroutine will
 provide either first or second order interpolation with derivative.
 FCTR: IN
 
 X - Independent variable array.
 FX - Dependent variable array.
 NPTS - Number of points in above arrays.
 X0 - Point for which the interpolation is to take place.
 FX0 - Interpolated value at X0.
 FPX0 - Derivative of the interpolation at X0.
 IORDER - Order of interpolation, either 1 or 2.
 I - Segment number containing X0.