Update to Iris 4.3
Based on Iris 4.3, apply commits of https://gitlab.mpi-sws.org/simonspies/iris-parametric-index/-/tree/simon/parametric-index?ref_type=heads to parametrize step-index.
Based on Iris 4.3, apply commits of https://gitlab.mpi-sws.org/simonspies/iris-parametric-index/-/tree/simon/parametric-index?ref_type=heads to parametrize step-index.