fCO2_recommended != NaN applied to selected variable instead of fCO2_recommended

[karlmsmith]

The LAS configuration for published SOCAT datasets include the constraints

      <constraints>
         <WOCE_CO2_water_cidx_0>2</WOCE_CO2_water_cidx_0>
         <fCO2_recommended_cidx_1>variable,ne,NaN</fCO2_recommended_cidx_1>
      </constraints>

but when a variable is selected from the dataset selector, the fCO2_recommended != NaN is applied to the selected variable instead of fCO2_recommended (such a salinity in the first plot - see plot annotations). If that variable is a string variable, an error message is returned (such as socat_version as mentioned in the error message in the second plot).

[karlmsmith]

Note that changing the variable from the list just above the plot preserves whatever != NaN constraint was originally applied.