Fix Doxygen warnings

[willat343]

Doxygen complains about some of the members for ArithmeticTypeTraits:

/home/william/workspaces/continuity_ws/src/mathbox/mathbox/include/mathbox/impl/traits.hpp:21: warning: no uniquely matching class member found for 
  template < Scalar, Rows, Cols, Options, MaxRows, MaxCols >
  constexpr auto math::ArithmeticTypeTraits< Scalar, Rows, Cols, Options, MaxRows, MaxCols >::zero() -> -> ArithmeticType

/home/william/workspaces/continuity_ws/src/mathbox/mathbox/include/mathbox/impl/traits.hpp:21: warning: no uniquely matching class member found for 
  template < Scalar, Rows, Cols, Options, MaxRows, MaxCols >
  constexpr auto math::ArithmeticTypeTraits< Scalar, Rows, Cols, Options, MaxRows, MaxCols >::zero() -> -> ArithmeticType