mf6 river test: test river package is initialized with scalar values

This should not work, even though it is included in a test:

riv = imod.mf6.River(stage=1.0, conductance=10.0, bottom_elevation=-1.0)

This is allowed for GRIDDATA variables (e.g. conductivity in the NPF package), but not for these list-based input packages such as RIV, DRN, GHB, etc.

Could be combined with type checking, since an array is a different type than a scalar.