fix issue #9 about square bracket

The toolkit handles CHEM monomers without using square bracket to
enclose monomer ID or extended SMILES, since simple CHEM polymer
contains only one monomer. Implemented add and remove square bracket
routines to handle notation I/O in editor. Input notation with or
without square brackets can loaded, while output HELM notation will
contain square bracket and output canonical HELM will not contain square
bracket.