Add subcell finite volume differencing and reconstruction operators

[jlchan]

Adds the multi-diimensional equivalent of the sparse subcell operators in Lin, Chan (2024) (see also https://doi.org/10.1016/j.jcp.2023.112677)

Also improves docs and robustness of sparse subcell SBP operators.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.48872% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 92.80%. Comparing base (2c56f42) to head (f387929). Report is 29 commits behind head on dev.

:exclamation: Current head f387929 differs from pull request most recent head c2e3654

Please upload reports for the commit c2e3654 to get more accurate results.

Files	Patch %	Lines
src/low_order_sbp.jl	95.83%	5 Missing :warning:
src/ref_elem_utils.jl	92.30%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #163 +/- ## ========================================== - Coverage 97.12% 92.80% -4.33% ========================================== Files 25 27 +2 Lines 3167 3766 +599 ========================================== + Hits 3076 3495 +419 - Misses 91 271 +180 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.