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.