feat: use pooch for data management

[nstarman]

Fixes #50.

Pooch works for all shell types (because it doesn't use shell commands).
Pooch will try multiple times if the connection is interrupted. It does file caching and re-downloading. The cache location is customizable. I made target_dir be a symlink, so the file is only ever downloaded once.

The Pooch registry is called strato because "Strato is a minor character in William Shakespeare's play Julius Caesar. Although he doesn't have a significant role, he serves as a loyal and trustworthy friend to Brutus" link

[nstarman]

I didn't implement it because I didn't want to change the API, but IMO strato should be imported into the top namespace, so one could do

from brutus import strato
from brutus import seds
from brutus import filters

isochrone = seds.Isochrone(
    filters=filt,
    nnfile=strato.fetch("nn_c3k.h5"),  # will auto-download if necessary
    mistfile=strato.fetch("MIST_1.2_iso_vvcrit0.0.h5"),
)

[nstarman]

I also didn't do it here, but with strato there's also not much point to all the fetch_X utilities... removing them would be another nice simplification. It does mean removing the symlink generation, but that's an easy single utility function

def fetch(name, symlink_dir):
    fpath = strato.fetch(name)
    fpath = pathlib.Path(fpath)
    
    target_path = pathlib.Path(symlink_dir).resolve() / name
    target_path.mkdir(parents_ok=True, exists_ok=True)
    target_path.symlink_to(fpath)
    return target_path