BUG: Add updated megares mappings

BUG: Add updated megares mappings that were missed by #79

- Add tests to check if megares mappings have been correctly derived from argnorm's argannot and resfinder mappings

These mappings are aggregated in `construct_megares_mapping.py` and are initially stored in './db_harmonisation/mapping/megares_resfinder_argannot_mapping.tsv`. This is then moved into `./tests/megaes_mappings/`