xvariable.select with an std::array

[martinRenou]

We should say in the documentation that an other set of curly braces is needed when using select specifying the size of the selector.

// Select with an std::vector -> initializer list
auto val = var.select<join::outer>({{"abscissa", "a"}, {"ordinate", 1}});

// Select with an std::array of size 2 -> additional curly braces 
//                                        around the initializer list
auto val = var.select<join::outer, 2>({{{"abscissa", "a"}, {"ordinate", 1}}});