specifying CORRECTED_DATA:V doesn't seem to work

[o-smirnov]

Same error for CORRECTED_DATA or CORRECTED_DATA/MODEL_DATA:

$ shadems msdir/4C12.03-610-cal.ms --field 3C48 -x BASELINE -y FREQ -a CORRECTED_DATA/MODEL_DATA:V --ared std --baseline S06-* --cnum 64 --cmap pride --dir plots/
...
ValueError: 'V' not applicable with column CORRECTED_DATA/MODEL_DATA

$ shadems msdir/4C12.03-610-cal.ms --field 3C48 -x BASELINE -y FREQ -a CORRECTED_DATA/MODEL_DATA:V:amp --ared std --baseline S06-* --cnum 64 --cmap pride --dir plots/
...
ValueError: function specified twice in 'CORRECTED_DATA/MODEL_DATA:V:amp'