Fix #475 : Added an option to refer a folder of GFF files

[IvanUgrin]

This patch adds an option to refer a folder of GFF files to the agat_sp_merge_annotations.pl script. It addresses feature request #475

[Juke34]

Thank you for your involvement.
Could you update the regex to recognize gtf and gff file?
If there is a way to minimize the external library to use it would be nice. If not the new libraries have to be added in the makefile (and in the bioconda recipe. This is for remember myself)

[IvanUgrin]

Hey, I've added GTF support and also removed the Basename import. I don't think I can remove the other two imports as they are needed to detect the files on a folder level.