Add numerical support of other real types (`linear_advection`)

[huiyuxie]

Continue #1909.

Linear advection seems easier to review and merge, so I do these first in equations.

Tasks:

• [x] linear_scalar_advection 1d, 2d, 3d
• [x] unit tests for above

[github-actions[bot]]

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.16%. Comparing base (6bb0be5) to head (23c0050).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1971 +/- ## ========================================== + Coverage 96.15% 96.16% +0.01% ========================================== Files 460 460 Lines 36949 36958 +9 ========================================== + Hits 35527 35539 +12 + Misses 1422 1419 -3 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1971/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/trixi-framework/Trixi.jl/pull/1971/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `96.16% <100.00%> (+0.01%)` | :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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[huiyuxie]

Okay

[huiyuxie]

Ready for final review @ranocha :/