Fix non-const Color::operator[] to return mutable reference#701

azeey merged 6 commits intogazebosim:maingazebosim/gz-math:mainfrom

nunoguedelha:nunoguedelha/fix/index-oper-to-ret-mutable-refnunoguedelha/gz-math:nunoguedelha/fix/index-oper-to-ret-mutable-refCopy head branch name to clipboard