Support structured binding of fixed-size vectors

[demianmnave]

It would be useful to support C++17 structured binding to the elements of a fixed-size cml::vector<>. For example:

auto [x,y,z] = cml::vector3d(1.,2.,3.);

should yield

x = 1.
y = 2.
z = 3.

[demianmnave]

Fixed in d351844