Add function `search_radius`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.75%. Comparing base (2faae44) to head (f6035f2).

Files	Patch %	Lines
src/nhs_precomputed.jl	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #35 +/- ## ========================================== - Coverage 91.26% 90.75% -0.52% ========================================== Files 15 15 Lines 355 357 +2 ========================================== Hits 324 324 - Misses 31 33 +2 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/35/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/35/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `90.75% <50.00%> (-0.52%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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