In sites.py, get_downloaded_sites() accepts a jsonurl for offline access (default is online). It is called by _get_site_registry() in earth.py with no way to change the default. _get_site_registry() in turn is called by of_site() also in earth.py, also with no way to change the default.

Since observatories don't move around every day, it is not unreasonable to make it easier for user to pass in jsonurl to of_site() if they wish to avoid network connection.

This was a follow-up for Scipy 2018. cc @stargaser , @eteq , and @adrn