Adding chemistry name helpers

[bmorris3]

These helper methods allow a user to enter a common atomic/molecular species name or an isotopologue name, and convert it to other naming conventions or find masses for atoms and molecules. These methods are useful for accepting a wide range of user input when specifying species for opacities and chemistry calculations.

[bmorris3]

I'm not sure what's up with your docs build in that screenshot. The remotely-built docs work fine (see build logs here).