Central SBP finite difference solver for UnstructuredMesh2D

[andrewwinters5000]

This implements a new volume integral and a few additional containers such that the FDSBP solver type works on curvilinear UnstructuredMesh2D. Two elixirs are added for the compressible Euler equations to test converges and free-stream preservation. This extracts the central SBP solver that is working from #1295 (as suggested in #1295 (comment)). There are still issues and open questions about the Upwind FDSBP on unstructured domains.

[github-actions]

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (034b158) 96.24% compared to head (37b4bd8) 92.44%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/solvers/fdsbp_unstructured/fdsbp_2d.jl	97.92%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1773      +/-   ##
==========================================
- Coverage   96.24%   92.44%   -3.80%     
==========================================
  Files         431      433       +2     
  Lines       34896    35049     +153     
==========================================
- Hits        33583    32400    -1183     
- Misses       1313     2649    +1336     

Flag	Coverage Δ
unittests	92.44% <98.76%> (-3.80%)	⬇️

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[ranocha]

Thanks! Please find some comments below.