Replace all references to "particle" by "point"

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.

Project coverage is 88.07%. Comparing base (591827b) to head (74edb3a).

Files	Patch %	Lines
src/nhs_grid.jl	86.36%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #31 +/- ## ======================================= Coverage 88.07% 88.07% ======================================= Files 11 11 Lines 260 260 ======================================= Hits 229 229 Misses 31 31 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/31/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/31/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `88.07% <94.33%> (ø)` | | 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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