ADAS Subroutine h4lnft

C
       subroutine h4lnft( istdim ,
     &                    x      , xsa   , y     , ysa   , ict
     &                  )
c-----------------------------------------------------------------------
c
c  **************** fortran77 subroutine: h4lnft.for *********************
c
c  purpose:  perform linear interpolation
c
c  
c  input : (r*8)  x     = required x-value
c  input : (r*8)  xsa(i)= x-values
c  input : (r*8)  ysa(i)= y-values
c  input : (i*4)  ict   = number of values
c   
c  input : (r*8)  y     = returned y-value
c
c  author:  Hugh P. Summers, University of Strathclyde
c           JA7.08
c           Tel.: +44 (0)141-548-4196
c
c  date:    24 July 2002
c
c
c  version: 1.1   Hugh Summers  24/07/02
c  modified:      first release
c-----------------------------------------------------------------------
      INTEGER             ICT,         ISTDIM
      REAL*8              X,           XSA(ISTDIM), Y
      REAL*8              YSA(ISTDIM)