This is an attempt trying to make #299 less breaking.

Previously in our gis examples, we demonstrated how to subclass Cell:

class LakeCell(mesa_geo.Cell):
    def __init__(self, model, pos=None, indices=None):
        super().__init__(model, pos, indices)
        ...

But this will result in an error TypeError: LakeCell.__init__() got an unexpected keyword argument 'rowcol' with the changes in #299.

Therefore in mesa/mesa-examples#320 we update our gis example to update Cell subclass to:

class LakeCell(mesa_geo.Cell):
    def __init__(self, model=None, **kwargs):
        super().__init__(model, **kwargs)
        ...

To make it less breaking, in this PR we update the _initialize_cells method in RasterLayer so that it works with both the old and the new ways of subclassing Cell.

In addition, we also removed the deprecation warning for cell.pos since it inherits Mesa.Agent and its pos is set to None by default, trigging the warning (see mesa/mesa-examples#320 (comment)).