TS plotter requires params.TEMPERATURE but uses params.POTENTIAL_TEMPERATURE

[eleanorfrajka]

TS plotter requires temperature:

    required_vars = [params.TEMPERATURE, params.SALINITY, params.DEPTH]

But if potential temperature exists, it'll use that instead:

  if params.POTENTIAL_TEMPERATURE in ds:
        temperature = ds[params.POTENTIAL_TEMPERATURE]

Since in the demo notebook, we have primary and secondary temperature temperature_1 and temperature_2, this required_vars fails, even though potential_temperature is present without the _1 subscript.

Better to update the required to require either params.TEMPERATURE or params.POTENTIAL_TEMPERATURE.