Ensure base URLs always end in /

Handles case where a user specifies a custom URL of something like
http://foo.com/ancillary which would expand to something like
http://foo.com/ancillary2024-08-05-file.data

https://gitlab.ssec.wisc.edu/cspp_geo/cspp-geo-aitf/-/issues/468

csppfetch/__init__.py

@@ -1083,6 +1083,7 @@ class Downloader:
             if ':/' not in u:
                 logging.warning(f"Assuming that {u} (from {envname}) should be file:///{u}")
                 u = "file:///"+u
+            if not u.endswith('/'): u += '/'
             checkedurls.append(u)
         return checkedurls