* moved all math constants from basic_math.hpp to const_math.hpp + these are variable templates now (names are the same for now, except for the *_MUL_PI ones which are now called PI_MUL_* for consistency)
 * removed the RAD2DEG and DEG2RAD macros and added proper functions in const_math (now named rad_to_deg and deg_to_rad)
 * removed the float comparisons with an epsilon macros and added proper functions in const_math (now named is_*)
 * killed of the basic_math.hpp header now that everything is implemented in const_math.hpp
 * matrix4 is now fully constexpr (except for ostream operator<< and to_string of course)
 * all vector assignment operators should be noexcept (probably not necessary, but hey ... not sure why there is no way to declare a complete class noexcept, or why anything constexpr isn't noexcept by default)