[MRG] add script for merging signatures by value in spreadsheet column

[bluegenes]

For example, to merge into a single signature per species:

python mass-merge.py -F gtdb-rs202.taxonomy.v2.csv gtdb-rs202.nucleotide-k31.zip \
-o gtdb-rs202.nucleotide-k31.species.zip --merge-col species

motivation:

for gtdb rs202 db

• ~70% of database belongs to species with >=10 genomes represented
• ~ 50% belongs to species w/ >=100 genomes represented.
  A merged db should save significant space by eliminating redundancy in hashes shared across strains and be good enough for many non-strain analyses.

7.7G full db --> 1.6G species database (k 31, scaled 1000)