Accept zero as quant values for protein groups

Default setting rejects quantification values of 0. Now changed to accept 0 as quantification value for protein groups. Do we really want to change this? Alternatively, we could remove all protein groups without quantification values (only for scaffold results or for all).

Changed snakemake version <7 in environment. snakemake >7 breaks the pipeline (Even in presence of all required database files, the rule download_database_file.smk is executed/tried to resolve). This throw an error in db_access.py __get_db_version_key() since a date in the past (from user config file or read from db_config.yaml files) is not valid for downloading protein sequence databases.