update luminosity function to work with minihalos

[codecov[bot]]

Codecov Report

Base: 86.33% // Head: 86.36% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (4ff8d85) compared to base (e271292). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #295 +/- ## ========================================== + Coverage 86.33% 86.36% +0.02% ========================================== Files 12 12 Lines 2780 2786 +6 ========================================== + Hits 2400 2406 +6 Misses 380 380 ``` | [Impacted Files](https://codecov.io/gh/21cmfast/21cmFAST/pull/295?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=21cmfast) | Coverage Δ | | |---|---|---| | [src/py21cmfast/outputs.py](https://codecov.io/gh/21cmfast/21cmFAST/pull/295/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=21cmfast#diff-c3JjL3B5MjFjbWZhc3Qvb3V0cHV0cy5weQ==) | `92.16% <100.00%> (+0.03%)` | :arrow_up: | | [src/py21cmfast/wrapper.py](https://codecov.io/gh/21cmfast/21cmFAST/pull/295/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=21cmfast#diff-c3JjL3B5MjFjbWZhc3Qvd3JhcHBlci5weQ==) | `89.06% <100.00%> (+0.05%)` | :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=21cmfast). 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=21cmfast)

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