VectorXYZT::mag() may be confusing since not four-vector magnitude

mag() is defined as

double mag() const {return std::hypot(x, y, z);}

but the magnitude of a fourvector is traditionally defined as mass() (maybe up to a sign)

double mass() const {return sqrt(t*t - x*x - y*y - z*z);}

This might lead to confusion when people start to use this in things like q2 = (ef-ei).mag().