Generalize disambiguation of CLI arguments

[YPares]

( This might be unnecessary if #47 is tackled )

For now, the flag names for the CLI are generated by inspecting each DocRec independently of the others. That means that if two DocRec of options at two different points in the pipeline both expose a field with the same name, only the first one will be exposed via CLI (the second is still settable in the config file, though).

There is a disambiguation method, but currently it works only at the level of one DocRec (if two fields, named with their two "paths" p1 and p2, are so that last p1 == last p2). This feature is rarely used because few tasks make use of the hierarchical nature of DocRec names (most use only one-level namings). We should extend that disambiguation process to the whole pipeline.