make it possible to give initial iteration a timestep number

...se-Richards/multi-patch/layered_soil/TP-R-layered_soil.py

...atch/TP-R-layered_soil_with_inner_patch-all-params-one.py

...ner_patch/TP-R-layered_soil_with_inner_patch-realistic.py

...ayered_soil_with_inner_patch-all-params-one-mesh-study.py

.../TP-R-layered_soil_with_inner_patch-pure-dd-mesh-study.py

...esh_study/TP-R-layered_soil_with_inner_patch-realistic.py

...e-with-inner-patch/TP-TP-layered_soil_with_inner_patch.py

...h_study/TP-TP-layered_soil_with_inner_patch_mesh_study.py