Move cell list management to a new struct

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.16%. Comparing base (989d0c0) to head (8f285b8).

Files	Patch %	Lines
src/nhs_grid.jl	88.88%	2 Missing :warning:
src/nhs_trivial.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12 +/- ## ========================================== + Coverage 85.02% 86.16% +1.13% ========================================== Files 9 10 +1 Lines 207 224 +17 ========================================== + Hits 176 193 +17 Misses 31 31 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/12/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/12/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `86.16% <90.90%> (+1.13%)` | :arrow_up: | 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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