[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.