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[bot]]

Codecov Report

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

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

:exclamation: 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

```diff @@ 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](https://codecov.io/gh/HaeffnerLab/IonSim.jl/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab) | Coverage Δ | | |---|---|---| | [src/IonSim.jl](https://codecov.io/gh/HaeffnerLab/IonSim.jl/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab#diff-c3JjL0lvblNpbS5qbA==) | `100.0% <ø> (ø)` | | | [src/ions.jl](https://codecov.io/gh/HaeffnerLab/IonSim.jl/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab#diff-c3JjL2lvbnMuamw=) | `87.8% <84.6%> (+1.5%)` | :arrow_up: | | [src/ionproperties.jl](https://codecov.io/gh/HaeffnerLab/IonSim.jl/pull/109?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab#diff-c3JjL2lvbnByb3BlcnRpZXMuamw=) | `97.3% <97.3%> (ø)` | | ... and [7 files with indirect coverage changes](https://codecov.io/gh/HaeffnerLab/IonSim.jl/pull/109/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=HaeffnerLab)

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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.