When returning a list of data-sources, Helioviewer.org uses the entries in the datasource table. It is possible, however,
that an entry may exist in the datasource table even when no actual data from that source is in the archive. Thus a method should be provided to ignore data-sources for which no data is available. This can occur either automatically during the getDataSources call (e.g. for each data-source, select count(*) where sourceId = n), or by adding an "enabled" column to the datasource table which could be used to toggle sources on or off.