Helpers for transparently downloading datasets

• fetch_file(download_url, filename = None, decompress = False, subdir = None)
• fetch_and_transform(transformed_filename, transformer, loader, source_filename, source_url, subdir = None)
• fetch_fasta_dict(download_url, filename = None, subdir = None)
• fetch_fasta_db(table_name, download_url, fasta_filename = None, key_column = 'id', value_column = 'seq', subdir = None)
• fetch_csv_db(table_name, download_url, csv_filename = None, subdir = None, **pandas_kwargs)