Refactor `psplups` treatment in `level_populations` and add test for full database

[wtbarnes]

Fixes #228 Fixes #227 Fixes #220

This PR does some minor refactoring to the level populations for handling the proton rates and also adds a test on the CI for the whole database.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.70 :warning:

Comparison is base (66a0a3e) 91.96% compared to head (0cc5397) 91.26%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #229 +/- ## ========================================== - Coverage 91.96% 91.26% -0.70% ========================================== Files 23 23 Lines 2015 2015 ========================================== - Hits 1853 1839 -14 - Misses 162 176 +14 ``` | [Impacted Files](https://codecov.io/gh/wtbarnes/fiasco/pull/229?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes) | Coverage Δ | | |---|---|---| | [fiasco/ions.py](https://codecov.io/gh/wtbarnes/fiasco/pull/229?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes#diff-Zmlhc2NvL2lvbnMucHk=) | `96.59% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://codecov.io/gh/wtbarnes/fiasco/pull/229/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes) 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=Will+Barnes). 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=Will+Barnes)

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