Build dictionaries from gtf file

[ns-rse]

Currently we have...

    tx2gene = defaultdict(list)
    strand_dict = defaultdict(str)

    for entry in GTF.iterator(iotools.open_file(argv_as_dictionary["gtf_path"])):
        if not entry.feature == "transcript":
            continue
        strand_dict[entry.gene_id] = entry.strand
        tx2gene[entry.gene_id].append(entry.transcript_id)

    conversion_counts = list()
    conversion_dict = defaultdict(int)

NB converion_counts and conversion_dict are assigned but never used so I don't think these are required.