Move URLs to its dedicated module

[rvhonorato]

Currently the API endpoints are spread troughout the code;

from arctic3d.modules.pdb import BESTPDB_URL, PDBE_URL
from arctic3d.cli_localise import UNIPROT_API_URL
from arctic3d.modules.interface import LIGAND_URL

It would be easier to maintain and check on them if there were in their own module