Improve functionality of atomic subpackage about plasmapy HOT 7 CLOSED

Here are some antimatter matters with inconsistent or undesirable behavior:

>>> charge_state('p-')
-1
>>> charge_state('e+')
ValueError: Invalid element or isotope information in charge_state
>>> charge_state('positron')
ValueError: Invalid element or isotope information in charge_state
>>> charge_state('antiproton')
ValueError: Invalid element or isotope information in charge_state

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We may want charge_state to allow electrons, even though it was intended for ions.

>>> charge_state('electron')
ValueError: Invalid element or isotope information in charge_state
>>> charge_state('e-')
ValueError: Invalid element or isotope information in charge_state

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Some more thoughts while I'm talking to myself: 😸 💬

• It would probably helpful to have a function called charge that would give the physical value of the charge for various particles. This would be easy to add.
• I'm wondering if it would be worth considering particles like muons, pions, etc. These do not come up that often for most plasma physicists, but may come up if people are studying cosmic rays and things like that and would also be straightforward to add.
• There are still lots of half-lives for different isotopes that are missing.

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It would be pretty handy to have charge_state and even ion_mass to allow electrons and perhaps more exotic particles - as it stands with Species in #54 , I would like to permit initializing those with a string and avoid having special cases for electrons. This seems like a better place for that.

A charge function seems trivial to pull off, as most of the exceptions are already in charge_state (did anybody say, low hanging fruit? 😃 )

Oh yeah, one more thing: I think we could do some automated testing here using pytest fixtures, given that all the data is already really neatly placed into python data structures.

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One more bug:

>>> plasmapy.constants.ion_mass('He-3 2+')
<Quantity 6.64465723e-27 kg>
>>> plasmapy.constants.ion_mass('He-4 2+')
<Quantity 6.64465723e-27 kg>

'He-3 2+' is giving the wrong mass, since it's interpreting it as an alpha particle.

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Of the bugs I mentioned, I believe all have been fixed except for this one:

>>> charge_state('H-1-')

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This was resolved in: #122

The original title of this issue (to allow ions to be represented as a tuple) did not get implemented since something like ('H', 1) would leave it ambiguous about whether the 1 meant the charge state or the isotope. The Species class is an improvement over this idea.

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