LEGEND Snakemake plugin

We should start to setup a Snakemake plugin soon. Wishlist:

- `touch` option to just replace every rule command/script with just `touch {output}`
- `execenv` command wrapping functionality
- setup `execenv` in profile files
- populate Numba cache on start, one-threaded
- write to scratch space and then move to the slow filesystem (check if there are existing solutions)