This attribute (together with SourceVRF, RefRA and RefDec) defines the
"Source" standard of rest (see attribute StdOfRest). This is a rest frame
which is moving towards the position given by RefRA and RefDec at a
velocity given by SourceVel (in km/s). When setting a value for
SourceVel, the velocity should be supplied in the rest frame
specified by the SourceVRF attribute. Likewise, when getting the
value of SourceVel, it will be returned in the rest frame specified
by the SourceVRF attribute.
The default value is zero.