Update GIS example models for Cell API changes

### Problem
Three GIS example models in Mesa-examples are failing due to breaking changes introduced in https://github.com/mesa/mesa-geo/pull/299, which added new keyword arguments (`rowcol`, `xy`) to the `Cell.__init__()` method.

**Failing models:**
- `gis/urban_growth` - `UrbanCell`
- `gis/rainfall` - `LakeCell`
- `gis/population` - `UgandaCell`

**Error:**
```
TypeError: .__init__() got an unexpected keyword argument 'rowcol'
```

### Root cause
The updated `Cell.__init__()` in mesa-geo now accepts keyword-only arguments `rowcol` and `xy`:

```python
def __init__(self, model, pos=None, indices=None, *, rowcol=None, xy=None):
```

Custom `Cell` subclasses in the example models don't accept these arguments, causing instantiation to fail when `RasterLayer._initialize_cells()` tries to create cells.

### Additional considerations
While fixing the signatures, we might update the models to use the new `cell.rowcol` property instead of the deprecated `cell.indices` where applicable.

Start with the GIS example entry points in gis/urban_growth, gis/rainfall, and gis/population, locating UrbanCell, LakeCell, and UgandaCell. Read how RasterLayer._initialize_cells() constructs these cells and compare the subclasses with the updated Cell.__init__() signature. Done means all three models instantiate successfully and applicable uses of deprecated cell.indices have been reviewed for cell.rowcol.

Written by the indexing model from the issue text.