Feat: create modelgrid without flopy; allow loading from .dis.grb

[mwtoews]

This PR does a few things:

• Adds a light-weight gridit.modflow.ModelGrid dataclass, which has a .to_grid() method to create a Grid instance; this class does not need flopy, unless using from_modflow or from_modelgrid classmethods.
• Allows reading MODFLOW 6 binary grid files (.dis.grb) with Grid.from_modflow() or ModelGrid.from_modflow()
• Deprecate get_modflow_model() with objects, as this method should read from PathLike or str to a MODFLOW model.
• Fix basic errors with the example MODFLOW 6 simulation