Adding config-style definitions for ion species properties

[psktam]

Summary:

• Adding config files for ion properties rather than defining them in code
• Removed the interstitial IonInstance class. We now directly instantiate Ion
• Moved IonProperties and config file functions to their own file, as it felt like ions.jl was getting too big.

[psktam]

Ah sorry @marwahaha ; didn't mean to tag you as a reviewer; it was a fat-finger, and it doesn't seem like I can remove you. Feel free to review or not review as you please.

[codecov]

Codecov Report

Patch coverage: 94.0% and project coverage change: +5.4 🎉

Comparison is base (3bbdce9) 83.5% compared to head (2861498) 88.8%.

❗ Current head 2861498 differs from pull request most recent head 4892a28. Consider uploading reports for the commit 4892a28 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #109     +/-   ##
========================================
+ Coverage    83.5%   88.8%   +5.4%     
========================================
  Files          15      13      -2     
  Lines        1488    1270    -218     
========================================
- Hits         1242    1128    -114     
+ Misses        246     142    -104     

Impacted Files	Coverage Δ
src/IonSim.jl	100.0% <ø> (ø)
src/ions.jl	87.8% <84.6%> (+1.5%)	⬆️
src/ionproperties.jl	97.3% <97.3%> (ø)

... and 7 files with indirect coverage changes

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[neil-glikin]

Have some convenient shorthand for loadfromconfig("../configs/ions/[ionname].yaml") to be able to easily load properties of predefined ion species

[jbroz11]

Have some convenient shorthand for loadfromconfig("../configs/ions/[ionname].yaml") to be able to easily load properties of predefined ion species

I think this is ready to merge once this is done.