COMPUTE DERIVATIVE

        [SIC\]COMPUTE OutVar DERIVATIVE InVar

    Compute the derivative of the InVar variable, in  the  classical  sense,
    i.e.
      OutVar[i] = InVar[i+1]-Invar[i]
    InVar  and OutVar are assumed to be 1-D variables, of size N.  OutVar[N]
    is linearly extrapolated from the N-1 and N-2 values.