getSDMXServiceProviders: getSDMXServiceProviders
Description
function used to get the list of SDMXServiceProvider known
by rsdmx (hence known by readSDMX to query data or
metadata in an easier way). This function can be easily used to
interrogate the list of known providers, and eventually consider
adding one at runtime with addSDMXServiceProviderUsage
getSDMXServiceProviders()