Need a way to store atomic data #18

We are frequently going to need to reference basic atomic data such as the atomic number and atomic mass, including for different ionization states and different isotopes. We will need this information for numerous applications, including as basic as getting the Alfven speed from the number density and the ion species. There will also be situations that require significantly more atomic data (e.g., calculating radiative cooling rates or ionization/recombination rates as a function of temperature/energy) so our approach should be able to be extended.

There do exist atomic databases with Python interfaces including ChiantiPy and pyatomdb, though the latter is in a pre-alpha state of development. We could look into how these databases store both basic and more detailed information, and perhaps using one of them as an optional dependency.

My questions are:

• How should we store atomic data?
• What atomic data should we store?
• How should we access stored atomic data? (e.g., what API should we use?)