update of fluxerror.tcl for multiple datasets

Shao Li noted that fluxerror.tcl doesn't work if multiple datasets have been read in. An immediate workaround is just to change "tcloutr flux" to "tcloutr flux 1" in the script. I have checked in a new version of the script which returns an error range for each dataset - note that the flux ranges will only actually be different if the datasets are members of different datagroups.