bug: FloPy load throws error reading .nam file for a USG model

[SamanTavakoli]

Describe the bug
FloPy throws an error attemping to load a USG model. The USG model has LPF package for model properties.

To Reproduce
ml = flopy.modflow.Modflow.load("SV_SWI_uni_01j.nam" ,version = "mfusg", verbose = True, check = False)

File D:\Software\Anaconda3\lib\site-packages\flopy\modflow\mf.py:811 in load
dis = disnamdata.package.load(

AttributeError: 'NoneType' object has no attribute 'load'

[cnicol-gwlogic]

I'll have to add a trap for this case, but the correct way to load mfusg models is:

flopy.mfusg.MfUsg.load()

Give that a shot.

[wpbonelli]

I think this can be closed, the deprecation of USG support was turned into an error with 696a209