config: Node object refactoring to allow on-demand references evaluation, and improvements on cloning and conversion to dictionary

* introduction of the Reference object
    - reference object has an 'eval' function to evaluate a reference
    - evaluation returns an object from config, or the value of an object attribute
* '$' prefix in YAML values now creates a Reference
    - no reference evaluation at YAML parsing time
* backward compatibility
    - getting a key with a reference returns the value, not the Reference
        * => introduction of ConfigList object to represent lists
    - '.raw_*' methods returns the raw contents, ie. with Reference objects instead of reference evaluated values
* node filenames are kept in Node objects
    - removed 'node2file' dictionary
* nodes are indexed automatically when inserting a 'name' key
    - no need for 'create_index'
    - same for tags
* '.to_dict()' method relies on the Python JSON decoder
    - simplification of the code
* '.deep_copy()' is renamed to ...