Implementation of a global gff parser
So far, the gff mechanism relies on the gff annotations of transcripts within a given sequence. Therefore, the parsing and tagging of the graph is highly specific to this kind of data, that is not acquirable for every organism.
In many cases, it could be important to use the gff annotation of the sequence we use in vizitig, and not only the gff with reference sequence of transcripts.
This change in mechanism cannot be detected so the implementation requires to add a new parameter to vizitig.