SMILES: add support for chirality specifiers and E-Z specifiers.

[electronicsbyjulie]

Chirality is specified using @ or @@ inside a bracketed expression. For example, a SMILES string for l-alanine would be:

N[C@@H](C)C(=O)O

Whereas d-alanine has only one @ and is otherwise the same.

E-Z isomerism is self explanatory; CC/C=C\CCO or CC\C=C/CCO is cis-3-hexen-1-ol or leaf alcohol; the trans isomer could be written either CC/C=C/CCO or CC\C=C\CCO.

[electronicsbyjulie]

One thing that will help:

Atom::swap_chirality is a public member variable that will be useful in interpreting @ and @@ symbols in SMILES.

[electronicsbyjulie]

mol_assem_test 'NC(C)C(=O)O'

...gives l-alanine.

mol_assem_test 'N[C@H](C)C(=O)O'

...currently throws an exception.

[electronicsbyjulie]

All of these work now:

mol_assem_test 'N[C@@H](C)C(=O)O'
mol_assem_test 'N[C@H](C)C(=O)O'
mol_assem_test 'CC/C=C/CCO'
mol_assem_test 'CC/C=C\CCO'
mol_assem_test 'CC\C=C\CCO'
mol_assem_test 'CC\C=C/CCO'
mol_assem_test 'CC/C=C\CCOC(=O)c1ccccc(O)1'