Allow the user to choose if re-using a SVI field or recalculating the SVI. GUI logic done. Missing calculation logic
Attempt to fix logic to allow the user to choose between re-calculating SVI or using an existing SVI attribute, while calculating the IRI
Remove combobox 'zonal attribute name' from attribute selection dialog, because it is not used anymore
Merge pull request #44 from gem/draw-iri-use-existing-svi

When calculating the IRI, allow the user to choose if re-calculating the SVI or using one of the available fields
Merge branch 'draw-iri' into draw-iri-improve-combining-operators

Resolved conflicts in:
	svir/calculate_iri_dialog.py
	svir/ui/ui_calculate_iri.py
	svir/ui/ui_calculate_iri.ui