fixed calculate weighted mean with 0 error, addressed issue #1318
fixed calculate weighted mean with 0 error, addressed issue #1318

UManPychron/pychron,NMGRL/pychron,USGSDenverPychron/pychron,UManPychron/pychron,NMGRL/pychron,USGSDenverPychron/pychron,USGSDenverPychron/pychron,NMGRL/pychron,NMGRL/pychron,UManPychron/pychron,USGSDenverPychron/pychron,UManPychron/pychron