cuboid::domain_t fix for std::vector constructor

This code previously couldn't be linked
`tqa::indexmaps::cuboid::domain_t<1> d({10});`
because of the corresponding constructor from std::vector using `static constexpr rank == Rank` in `triqs/arrays/indexmaps/cuboid/domain.hpp`. The constructor is defined inside the class definition, and so has the problem similar to the one, listed below.
See http://stackoverflow.com/questions/8016780/undefined-reference-to-static-constexpr-char
It works, if one uses the name from the template declaration.