function that allows searching by provider id in the list of
known SDMXServiceProvider. This function can be used for
interrogating the list of default providers known by rsdmx,
and is used internally by readSDMX
Usage
findSDMXServiceProvider(agencyId)
Value
an object of class "SDMXServiceProvider" (or NULL if no matching)
Arguments
agencyId
an object of class "character" representing a provider
identifier