sketch,build sbts w sourmash 4.0, multiple alphabets

[bluegenes]

Here's a Snakefile for building gtdb (or other) sbts using sourmash 4.0 sketching & indexing. Also added an environment.yml with environment than can be used to run this snakefile.

Configfile requires (default index_config.yml):

• genome and/or protein csv with at minimum, accession, filename columns
  • (provide signame for better signature naming)
• data-dir: where to look for the genome/proteome files
• specify only desired alphabet-ksize-scaled combinations for building sbts.

To run on different data or with different parameters, modify index_config.yml or provide a different configfile.

Alphabet defaults are taken from sourmash-bio/sourmash#1168.
Workflow does not generate translated sketches, as we don't want to build databases from 6-frame translations.

I also modified the farm sbatch files, and have corresponding snakemake profiles to use, but not sure we want to add those to this repo.

Likely needs modification (e.g. batching?) for even more genomes or additional parameters (bloom filter size?), but here's a starting point for 4.x.

[bluegenes]

Closing in favor of #16