Outputting of atom graph

[chrisiacovella]

To aid in setting up SMARTS for force fields, it would be useful to have a function that will take a molecule and spit out the local environment of an atom. @mattwthompson suggested we provide "guesses" of the SMARTS that would be sufficient to describe this as well. By guesses we mean SMARTS that are valid, but may not be specific enough to uniquely describe the atom type.

For all outputs, it would be good to output in multiple forms, e.g., element, numerical id, and if atom_type (if a file with known atom types is loaded in).

Related, this information should be given in the warnings if an atom type is not found.