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.