include/util/math/quaternion.h: refactor some member names to enable to other upcoming refactors
include/util/math/quaternion.h: switch camel cases method names to underscore; update usages
src/util/math/quaternion.cpp: be explicit with usage of the this keyword

 - we also change Quaternion Quaternion::operator*(const Quaternion& other) to avoid some confusion with scoping/variable names