Replace IVORN with IVOID #51
Description
IVOA Identifiers, version 2.0 deprecates the term "IVORN" in favour of IVOID.
The VOEvent standard continues to refer to IVORNs, as does the VOEvent XML schema.
Comet's documentation & command line options should refer to IVOIDs, but provide an explanation. Internally, we can continue to refer to IVORNs, since that's what we'll actually read out of packets.