Things waiting to be done:
- table with platforms
- references in web-documentation
- use std::numeric_traits for our constants Min_float, etc.
- linkage of our constants Min_float, etc.?
- use is_negative(), is_zero(), etc. instead of ordinary
  "<FT(0)", "==FT(0)", etc. statements
- make use of CGAL::square()
- move implementation details (like inner_product_n) to
  namespace Min_sphere_of_spheres_impl


Possible improvements:
- Better assertion and warning support for CGAL