* added misc math functions to the vector class (sqrt, inv_sqrt, sin, cos, ...) that can be applied to all vector components
 * added to_deg/to_rad functions to the vector class (convert vector components between degrees and radians)
 * changed all uses of vector_helper<scalar_type> to vector_helper<decayed_scalar_type> so that it should work with references now
 * added exp and log function to vector_helper and const_math_select
 * cleaned up the bool specialization in vector_helper a little (most of these functions make no sense when using a bool type)