Rewrite of 'AASequence::parseString_', increasing robustness of parsing (fixes #818)

Made parsing of amino acid sequences shorter, more modular and less convoluted. Also added "permissive" flag (on by default) to allow robuster parsing (issue #818).

'parseString_' uses the approach of 'parseUnmodifiedString_' (by @timosachsenberg) of querying ResidueDB to check if a character is an amino acid