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.
return the string identifying this orbit, assuming the context is provided elsewhere. For example, supposing this toString() is "orbit:rbspa-pp:43" then this would return just "43"