ADAS Subroutine b2nfit

       SUBROUTINE B2NFIT(X,XA,N,YAA,Y,DY,I0,C1,C2,C3,C4,ISW)            
C
       IMPLICIT REAL*8(A-H,O-Z)                                         
C-----------------------------------------------------------------------
C
C  ****************** FORTRAN77 SUBROUTINE: B2NFIT *********************
C
C PURPOSE:
C  SUBROUTINE TO PERFORM SPLINE INTERPOLATION                           
C
C
C  INPUT
C      X       = REQUIRED X-VALUE
C      XA(I)   = X-VALUES
C      N       = NUMBER OF VALUES
C      YAA(I)  = Y-VALUES (POSSIBLY STORED AS MULTIPLE SETS)
C      I0      = STARTING INDEX(-1) IN YAA ARRAY OF REQUIRED INPUT SET
C      C1(I,J) = 1ST SPLINE COEFFICIENT PRECURSOR
C      C2(I,J) = 2ND SPLINE COEFFICIENT PRECURSOR
C      C3(I,J) = 3RD SPLINE COEFFICIENT PRECURSOR
C      C4(I,J) = 4TH SPLINE COEFFICIENT PRECURSOR
C      ISW     = .LE.0  ORDINARY     SPLINE INTERPOLATION
C              = .GT.0  LOGARITHMIC  SPLINE INTERPOLATION
C  OUTPUT
C      Y       = RETURNED Y-VALUE
C      DY      = RETURNED DERIVATIVE
C
C NOTES: THIS ROUTINE IS NOT YET PROPERLY ANNOTATED
C
C UNIX-IDL PORT:
C
C VERSION: 1.1                          DATE: 06-03-96
C MODIFIED: TIM HAMMOND (TESSELLA SUPPORT SERVICES PLC)
C               - PUT UNDER S.C.C.S. CONTROL
C
C-----------------------------------------------------------------------
C
      INTEGER             I0,          ISW,         N
      REAL*8              C1(10,9),    C2(10,9),    C3(10,9)
      REAL*8              C4(10,9),    DY,          X,           XA(10)
      REAL*8              Y,           YAA(10)