Fixed TypeError in fill_holes_within_poly() in case older pandas version is installed.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>

py_tools_ds/geo/vector/topology.py

@@ -187,7 +187,7 @@ def fill_holes_within_poly(poly):
         gdf['area_filled'] = gdf.apply(
             lambda GDF_row: Polygon(np.swapaxes(np.array(GDF_row.geometry.exterior.coords.xy), 0, 1)).area, axis=1)
 
-        largest_poly_filled = gdf.loc[gdf['area_filled'].idxmax()]['geometry']
+        largest_poly_filled = gdf.loc[gdf['area_filled'].astype(np.float64).idxmax()]['geometry']
 
         # noinspection PyTypeChecker
         gdf['within_or_equal'] = gdf.apply(