Use contiguous backend for `FullGridCellList`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.12%. Comparing base (7d3c042) to head (96bf36a).

Files	Patch %	Lines
src/nhs_grid.jl	90.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #40 +/- ## ========================================== + Coverage 90.75% 91.12% +0.36% ========================================== Files 15 15 Lines 357 383 +26 ========================================== + Hits 324 349 +25 - Misses 33 34 +1 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/40/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/40/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `91.12% <97.87%> (+0.36%)` | :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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