Add a "category" template type

[JohanMabille]

The dictionary_encoded_layout can be used with types whose defualt layout is NOT the dictionary_encoded_layout: integers ( associated with fixed_size_layout), strings (asociated with variable_size_binary_layout), ...

This makes the creation of an array with this layout very cumbersome. A way to fix it is to create a new "category" template type, whose default associated layout is the dictionary_encoded_layout. This type should be a simple wrapper of its parameter, and is should be implicitly convertible to its parameter type.

An additional constructor may be added to the array if the category has to be entirely specified before building the array (simpler to implement but less flexible).

Plugging this "category" template type requires #148 to be implemented first.