Split implementation to separate header files

Rename classes to CamelCase and move them to the detail namespace:
  - uniform_elements_along to detail::UniformElementsAlong
  - uniform_groups_along to detail::UniformGroupsAlong
  - uniform_group_elements_along to detail::UniformGroupElementsAlong
  - uniform_elements_nd to detail::UniformElementsND
  - independent_groups_along to detail::IndependentGroupsAlong
  - independent_group_elements_along to detail::IndependentGroupElementsAlong

Introduce helper functions with the old names.