Make CONTROS Zero Flushing Time a Calculation Coefficient (was: CONTROS fCO2 not being calculated)

[squaregoldfish]

DFN and PALOMA

[squaregoldfish]

Issue was that the Zero Flushing Time was too long, so all the Zero values were set to FLUSHING and there were no records left for calibration.

This value should be made into a Calculation Coefficient, so users can edit it.

We also need documentation/a tutorial to help people understand what this and the Response Time do (along with anything else - ask Nadja and Vlad).

[squaregoldfish]

At the moment, changing the Zero Flushing Time requires deleting the dataset. We'll need to make sure that's not the case.