makes product iter assignable
makes product iter assignable

by removing a const data member

tempbottle/cppitertools,jkhoogland/cppitertools,jkhoogland/cppitertools,tempbottle/cppitertools,jkhoogland/cppitertools,ryanhaining/cppitertools,ryanhaining/cppitertools,tempbottle/cppitertools,ryanhaining/cppitertools