ENH: OTB-626 : use same formula as in fusion_phr.py, clean code
ENH: OTB-626 : use same formula as in fusion_phr.py, clean code

orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB,orfeotoolbox/OTB