#17 Molecule properties

[Cousteau]

It would be useful to not only be able to get the atomic mass of an atom, but also of a molecule. This way, you'd write something like `molecule("C12H22O11", "mass")` to get "342.296 u". This is, parse the string to get the atom symbols and indexes, and multiply each atomic weight by its index and sum. Additionally, there could be a small molecule database with some names (for example, "glycerin" instead of "C3H8O3") and properties (formation enthalpy and entropy, melting point...)