Speed up calculation of subcell bounds

[bennibolm]

Similar to #2644, this PR refactors the computation of subcell bounds in calc_bounds_twosided!. Instead of going through all nodes and decide whether there is an available neighbor (if i>1, if i<nnodes, ...), we run over all subcell interfaces (for i in 2:nnodes(dg), ...).
main

julia> @benchmark Trixi.calc_bounds_twosided!($var_min, $var_max, $variable, $u, $t, $semi, $equations)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  236.021 μs … 318.680 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     238.703 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   239.852 μs ±   3.522 μs  ┊ GC (mean ± σ):  0.00% ± 0.00%

        ▄▇█▆▄▅▅▅▃▂▂▂▁                         ▁                 ▂
  ▂▂▃▅▆██████████████████▇▇█▇▇█▇▇▆▇▆▆▆▇▆▅▆▇▆▆███▇▇▆▆▅▆▆▆▆▆▅▄▅▄▄ █
  236 μs        Histogram: log(frequency) by time        255 μs <

 Memory estimate: 0 bytes, allocs estimate: 0.

this PR

julia> @benchmark Trixi.calc_bounds_twosided!($var_min, $var_max, $variable, $u, $t, $semi, $equations)
BenchmarkTools.Trial: 10000 samples with 1 evaluation per sample.
 Range (min … max):  128.819 μs … 174.041 μs  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     131.858 μs               ┊ GC (median):    0.00%
 Time  (mean ± σ):   132.633 μs ±   2.932 μs  ┊ GC (mean ± σ):  0.00% ± 0.00%

     ▁▂▃▄▅▆▇██▇▇▆▅▅▄▄▃▂▂▁▁ ▁      ▁                             ▃
  ▄▆▇███████████████████████████████▇█▇████▆▇█▇▇█▇▇▇▇▆▆▇▇▆▆▆▆▆▆ █
  129 μs        Histogram: log(frequency) by time        145 μs <

 Memory estimate: 0 bytes, allocs estimate: 0.

I tried the same for the computation of one-sided bounds (nonlinear variables), but it makes the routine slower due to more evaluations of variable(..., equations).

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.01%. Comparing base (a499f99) to head (437ed1b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2727   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files         556      556           
  Lines       43958    43960    +2     
=======================================
+ Hits        42645    42647    +2     
  Misses       1313     1313           

Flag	Coverage Δ
unittests	97.01% <100.00%> (+<0.01%)	⬆️

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