DatumRange implementation that identifies times by orbit number. Note orbit numbers are strings, see Cassini for why.
next will return the same one at the end of the sequence.
dr= new OrbitDatumRange( "crres", "6" )
dr.toString() → "orbit:crres:6"
dr= dr.next()
dr.toString() → "orbit:crres:7"
Also, orbit:http://das2.org/wiki/index.php/Orbits/crres:6 is supported for development work and personal lists.