feature: Option to Associate Separate IMS Packages for GWF, GWE, GWT models in mfsim.nam

[MickeyRush]

The latest MODFLOW release checks for separate IMS packages in GWF-GWE simulations:
ERROR REPORT:

1. Problem with GWF-GWE exchange GWF-GWE_1. The GWF model must be solved
   by a different IMS than the GWE model. Furthermore, the IMS specified
   for GWF must be listed in mfsim.nam before the IMS for GWE.

As far as I can tell, flopy does not currently have a way to associate separate IMS packages with different models, so the mfsim.nam file ends up like:
BEGIN solutiongroup 1
ims6 ccg_gwf.ims ccg_gwf ccg_gwe
END solutiongroup 1

So far I have been manually editing my mfsim.nam:
BEGIN solutiongroup 1
ims6 ccg_gwf.ims ccg_gwf
ims6 ccg_gwe.ims ccg_gwe
END solutiongroup 1

But it would be nice to have an option in the flopy.mf6.ModflowIms() or flopy.mf6.ModflowGwfgwe() to associate those separate IMS packages with different models. Apologies if it already exists and I missed it.

[wpbonelli]

@MickeyRush this can be done with MFSimulation.register_ims_package(), see example here.