Support passing model(s) to solver packages

[wpbonelli]

Suggested by @mbakker7 in modflowpy/flopy#2355 and discussed further in modflowpy/flopy#2354. Though solvers live at simulation scope they are associated with (one or more) models. In flopy 3.x the registration step is awkward. It would be nice to be able to create this association by passing model(s) to the solver initializer. The models' association with a simulation then being transferred to the solver instead of requiring separate registration or both sim and model at solver init.

Currently we support a syntax which is kind of as requested, but still awkward as it expects string model names, not models themselves.

pyphoenix-project/test/test_mf6_component.py

Line 371 in d77b68a

ims = Ims(models=[gwf_name])

Maybe worth supporting both/either? Like

gwf = Gwf(name="mymodel", ...)
ims = Ims(models=[gwf], ...) # by model object
ims = Ims(models=["mymodel"], ...) # by model name

Loosely related to #259