move Get_arithmetic_kernel specialization into MP_Float.h reorder declaration of to_double(Sqrt_extension<MP_Float>) Sqrt_extension need specialization of Real_embeddable_traits and Algebraic_structure_traits.
move Get_arithmetic_kernel specialization into MP_Float.h reorder declaration of to_double(Sqrt_extension<MP_Float>) Sqrt_extension need specialization of Real_embeddable_traits and Algebraic_structure_traits.