Multipolygons cause "UnboundLocalError: local variable 'x' referenced before assignment" in rs cover

Nobody has claimed this yet.

I am just leaving this here as a note to anyone getting this error when using rs cover. If there are features in the cover GeoJSON that are of `MultiPolygon` type, they will throw the error in the title due to a limitation in supermercado, a dependency of robosat. Specfically, this function in "burntiles.py":

```
def _feature_extrema(geometry):
if geometry["type"] == "Polygon":
x, y = zip(*[c for part in geometry["coordinates"] for c in part])
elif geometry["type"] == "LineString":
x, y = zip(*[c for c in geometry["coordinates"]])
elif geometry["type"] == "Point":
x, y = geometry["coordinates"]
return x, y, x, y

return min(x), min(y), max(x), max(y)
```

You can get around it by making sure that your training polygons are forced to be `Polygon` and not `MultiPolygon` type before writing them to your cover CSV.

Start with burntiles.py and the _feature_extrema function used by rs cover, then determine whether the limitation belongs in robosat or its supermercado dependency. Reproduce the UnboundLocalError with a MultiPolygon cover GeoJSON and verify that rs cover handles it without the error while preserving the existing Polygon behavior.

Written by the indexing model from the issue text.