manage long N intergenic regions

[jwollbrett]

Some reference intergenic regions are full of N bp.
For instance for human, more than 80 reference intergenic regions are a sequence of 20.000 N.
As we provide reference intergenic sequences in our FTP for the BgeeCall package, we should remove these sequences.
We should maybe also remove all long N regions in intergenic regions.
One solution could be to remove all N regions bigger or equal to default kmer size of kallisto (31bp).
One initial 20.000bp reference intergenic region could then result to more than one reference intergenic region.