When calculating the IRI, allow the user to choose if re-calculating the SVI or using one of the available fields

A new checkbox ("recalculate the SVI") is added to the GUI in the calculate indices dialog, available when the "calculate IRI" checkbox is checked.
Cases:
* "recalculate" is checked: the comboboxes for the selection of the operators are enabled
* "recautulate" is not checked: a combobox for the selection of the attribute containing the SVI is enabled
The logic workflow of the calculation is modified accordingly.

When aggregating loss by zone, the combobox used to pick the zonal attribute name is not needed anymore, so this PR removes it and fixes the logic behind it.