ESSPRK Stepper does not accept final specifier with HPX

[kazbekkm]

ESSPRK Stepper class should be final, but compiling with HPX give the following error if final specifier is added to the class:

error: cannot derive from 'final' base 'ESSPRKStepper' in derived type 'hpx::traits::detail::has_hpx_serialization_get_name_detail::helper_composed'
HPX_HAS_MEMBER_XXX_TRAIT_DEF(hpx_serialization_get_name)