Fix free-bound continuum function

[wtbarnes]

Fixes #42

This PR is an effort to fix the inconsistency between the free-bound continuum implementation in fiasco versus is what is implemented in CHIANTI IDL. This also adds a suite of tests for comparing against the IDL implementation.

TODOs:

• [x] Update free-bound regression tests
• [x] Add tests for comparing free-bound per ion against IDL results
• [x] Finish docstring for free-bound method on Ion
• [x] Update docstring for free-bound method on Collection

Current progress:

[wtbarnes]

I found two bugs that have FINALLY resolved these inconsistencies:

• The free-bound continuum was being incorrectly multiplied by the population fraction of the recombined ion. It is now corectly multiplied by the recombined ion.
• The free-bound level energies were not being correctly filled in by the theoretical energies when the observed energies were missing

The updated comparison now looks like the following:

[codecov-commenter]

Codecov Report

Base: 86.92% // Head: 87.03% // Increases project coverage by +0.11% :tada:

Coverage data is based on head (bdaf496) compared to base (dc309d3). Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #196 +/- ## ========================================== + Coverage 86.92% 87.03% +0.11% ========================================== Files 22 22 Lines 1812 1820 +8 ========================================== + Hits 1575 1584 +9 + Misses 237 236 -1 ``` | [Impacted Files](https://codecov.io/gh/wtbarnes/fiasco/pull/196?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes) | Coverage Δ | | |---|---|---| | [fiasco/collections.py](https://codecov.io/gh/wtbarnes/fiasco/pull/196/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes#diff-Zmlhc2NvL2NvbGxlY3Rpb25zLnB5) | `70.96% <0.00%> (ø)` | | | [fiasco/ions.py](https://codecov.io/gh/wtbarnes/fiasco/pull/196/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes#diff-Zmlhc2NvL2lvbnMucHk=) | `89.95% <93.75%> (+0.45%)` | :arrow_up: | 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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