Add unweighted sum combination type
Add to combination types also the simple (unweighted) multiplication
Define default combination type in globals and import that everywhere it's used, instead of duplicating
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
Use default combination type also where it was missing
Merge pull request #46 from gem/draw-iri-improve-combining-operators

Add some combining operators