Treat all files as composites to simplify grammar

[raquelalegre]

At the moment, composites are only discovered when a new &-line is found in a text. At this point, the Text object created so far, has to be adapted to be a composite, and that's a little messy.
This could be improved if we consider all files as composites. Non-composite files would be composites with only one text element.

Right now, the entry point of the grammar is:

        document : text
                    | object
                    | composite

It should be changed to:

        document : composite
        composite : text 
                    | object
         text: AMPERSAND ......... 

This is not a trivial change and requires close attention and significant development time. It is not high priority, though.