Queries the back-end to retrieve a list of services that the current user owns. Services are
web services like WCS, WFS, etc. The result is an object of type ServiceList, which is a named list of Service. The
indices are the service IDs, the service object that is indexed by its ID and may use other functions instead of its service ID.
list_services(con = NULL)connected and authenticated openEO client object (optional) otherwise active_connection()
is used.
named list of Services (class ServiceList)