Reduce size argument could return compressed PGDB.

[ArnaudBelcour]

For input:

Folder_input
├── species_1
│   └── species_1.gbk
├── species_2
│   └── species_2.gff
│   └── species_2.fasta
├── species_3
│   └── species_3.gbk

Now, when using -r (or size_reduction), mpwt will create zip compressed files in the output folder:

Folder_output
├── species_1.zip
├── species_2.zip
├── species_3.zip

Also fix an issue with -r/size_reduction argument if not used with -o/output_folder.

[ArnaudBelcour]

Modify in #27.