Test positivity at postfit time and with grids instead of fktables

[scarlehoff]

Instead of using positivity fktables, at the postfit stage, we can use grids (that are then committed to this repository).

This is not a very complicated task but still complex enough that might need perhaps a week or two of dedication to complete it. I'm assigning you to it @juanrojochacon as you had interest on this in case you can spare someone at some point (I don't think it is very urgent).

Otherwise this might be a good first task for someone joining either as part of a master thesis or next year.

[juanrojochacon]

Received in good order. We can take a look at this with @jekoorn and @kamillaurent once we are done with the other ongoing actions.