Improve CHARMMTopology.add_sequence() to accept full residue names

[benmwebb]

Currently add_sequence() only accepts single-letter residue codes, so it can only really be easily used to construct amino acid sequences. Add support for mmCIF-style full residue names (in parentheses) so we can unambiguously construct DNA, RNA, etc., e.g. add_sequence('ACG/(ADE)(CYT)(GUA)/(DADE)(DCYT)(DGUA)') to add an amino acid, an RNA chain, and a DNA chain.