Improve robustness: check current axes

Without the assert, if the current axes is not a Geoaxes instance,
there is a less clear error with a plot instruction later.

Analysis/plot_snapshot.py

@@ -26,6 +26,7 @@ import matplotlib.pyplot as plt
 import math
 import cartopy.crs as ccrs
 from os import path
+from cartopy.mpl import geoaxes
 
 def snapshot(k, shp_tr_dir, *, dashed = False, light = False,
              new_figure = False, src_crs = ccrs.PlateCarree(),
@@ -73,6 +74,7 @@ def snapshot(k, shp_tr_dir, *, dashed = False, light = False,
         plt.suptitle(f"k = {k}")
     else:
         ax = plt.gca()
+        assert isinstance(ax, geoaxes.GeoAxes)
 
     for ishape in range(ishape_first, ishape_last[k - k1] + 1):
         shape_rec_extr = reader_extr.shapeRecord(ishape)