Atomic physics - #24

I wrote up a bunch of routines to access atomic data, including elements and isotopes. I figured that plasma physicists occasionally care about atomic physics, and then I might have gotten just a little bit ridiculous. For plasma parameters calculations, we'll need ion_mass. I had fun writing the tests too, as there were a bunch of special cases and I wanted to test the exceptions too.

The routines are:

• element_symbol
• isotope_symbol
• atomic_number
• is_isotope_stable
• half_life
• mass_number
• element_name
• standard_atomic_weight
• isotope_mass
• ion_mass
• nuclear_binding_energy
• energy_from_nuclear_reaction
• known_isotopes
• common_isotopes
• stable_isotopes

Remaining issues are:

• The inputs for these routines are quite flexible. element_symbol for example can accept inputs like 'hydrogen', 'H-1', 'D', 42, '42', 'protium', etc. These are case insensitive except where case makes a difference: 'p' refers to protons but 'P' refers to phosphorus. (Also apparently are are elements besides hydrogen. Who knew?) I included neutrons in a bunch of these too, though we might want to adjust whether we want functionality like element_name('n') returning neutron.
• I need to do a better job of documenting where the atomic data came from. Most of it came from NIST.
• I am not sure what the best way to store atomic data is. For this pull request, I wrote python code to write python code for dictionaries of dictionaries called Elements and Isotopes that contain rudimentary atomic data. I also don't know where the best place is to deposit helper code that I used to write the code for the Isotopes data structures.
• The half-life data is woefully incomplete, and I haven't found a good source. There are also complications that some isotopes have more than one half-life if the nucleus has multiple configurations. This made my brain hurt.
• I should write a quick routine isotopic_composition() to get the terrestrial relative abundances.
• I am wondering if these routines should be put in a different directory like physical_data instead of constants.
• Probably the best way to make sure code is most readable is to use atomic and isotopic symbols consistently, like atomic symbol for an element and 'Pb-209' for an isotope.
• I should look through this again in case there are any sections of the code that require some comments explaining what is going on (like # magic or # here be dragons), or if I should do some refactoring.
• There's a huge amount of atomic data that plasma physicists need. We will may want to partner with databases like CHIANTI and AtomDB eventually.
• My cat says hi.