Update tile retreival ahn.py (#371)

* Update tile retreival ahn.py

- On the eclipse server: Skip tile retreival if the link does not exist.
- Legacy server: Rasterio merge returned only zero's since 17 sept. All the merging is now done within xarray.

* Pleasing codacy

* Revert changes to legacy functions

nlmod/read/ahn.py

@@ -580,6 +580,8 @@ def _get_ahn_ellipsis(extent, identifier="AHN5_5M_M", **kwargs):
     das = []
     for tile in tqdm(tiles.index, desc=f"Downloading tiles of {identifier}"):
         url = tiles.at[tile, identifier]
+        if url == "nan":
+            continue
         if url.endswith(".zip"):
             path = url.split("/")[-1].replace(".zip", ".TIF")
             if path.lower().endswith(".tif.tif"):