Peptacular v2.2.0

• condense_mods function to condense_to_mass_mods.py
• [potentially breaking] updated digestion and span functions to return generator objects
• added sequential_digest, digest_from_config and EnzymeConfig to digest.py
• regex strings which don't have the same start/end site will give a warning such as ([KR])
• fixed regex bug with nterm enzymes
• simplified supported enzyme regexes
• added simple fasta_parser since I kept recreating it in other projects, works with several input types
• some more tests
• added condense_to_mass_mods to mass_calc.py which condenses modifications to a single +/- mass value
• Fixed Docs
• Linting