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]

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.

❗ 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 ⚠️
src/ref_elem_utils.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            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     

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