source type, e.g. "IEA". A list of all available source types can be retrieved with
function getSources("download").
subtype
For some sources there are subtypes of the source, for these source the subtype can be specified
with this argument. If a source does not have subtypes, subtypes should not be set.
overwrite
Boolean deciding whether existing data should be overwritten or not.
numberOfTries
Integer determining how often readSource will check whether a running download is finished
before exiting with an error. Between checks readSource will wait 30 seconds. Has no effect if the sources that
should be read are not currently being downloaded.