Add two-photon continuum method

[wtbarnes]

Fixes #43

[codecov-commenter]

Codecov Report

Base: 90.17% // Head: 90.87% // Increases project coverage by +0.70% :tada:

Coverage data is based on head (fb6f658) compared to base (40a32ad). Patch coverage: 66.66% of modified lines in pull request are covered.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #217 +/- ## ========================================== + Coverage 90.17% 90.87% +0.70% ========================================== Files 23 23 Lines 1882 1885 +3 ========================================== + Hits 1697 1713 +16 + Misses 185 172 -13 ``` | [Impacted Files](https://codecov.io/gh/wtbarnes/fiasco/pull/217?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/217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes#diff-Zmlhc2NvL2lvbnMucHk=) | `92.73% <66.66%> (-0.20%)` | :arrow_down: | | [fiasco/util/setup\_db.py](https://codecov.io/gh/wtbarnes/fiasco/pull/217?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Will+Barnes#diff-Zmlhc2NvL3V0aWwvc2V0dXBfZGIucHk=) | `80.21% <0.00%> (+15.38%)` | :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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