Pull inputs in run(), not run_as_update()

For years, when invoking geomag-algorithms' update mechanism, inputs were pulled and checked from the run_as_update() method inside the Controller class. I never understood this. It seemed to break the logic of the update mechanism if no inputs were available for the current inteval/gap. This might even be the source of noted issues where running back-filling scripts didn't behave as expected, and those scripts needed to be run multiple times.

What's more, the run_as_update() method logically seems like the most appropriate place to read outputs and check for gaps. Whereas the run() method seems like the most appropriate place to read inputs, apply algorithms, and write out outputs.

In any case, this change should not break any existing code. It should only allow the update mechanism to complete, every time, and as originally intended, rather than be short-circuited when input data are missing for the current interval (but might be available for previous intervals).

(mentioning @bgeels to get more eyes on this)