Ensure that all diff operators appear once with their argument at current
period (i.e. maxLag=0).
If it is not the case, generate the corresponding expressions.

This is necessary to avoid lags of more than one in the auxiliary
equation, which would then be modified by subsequent transformations
(removing lags > 1), which in turn would break the recursive ordering
of auxiliary equations.

See McModelTeam/McModelProject/issues/95 for an example.