Set the errors for the RDCs.
align_id: The optional alignment ID string.
spin_id1: The optional spin ID string of the first spin.
spin_id2: The optional spin ID string of the second spin.
sd: The RDC standard deviation value in Hertz.
If the RDC errors have not already been read from a RDC data file or if they need to be changed, then the errors can be set via this user function.