Avoid slicing in BCs since it leads to allocations

[DanielDoehring]

See #1594, adapt the same changes now also to the P4est examples

[github-actions]

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +8.62% 🎉

Comparison is base (bc67361) 87.51% compared to head (07d89b8) 96.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1637      +/-   ##
==========================================
+ Coverage   87.51%   96.13%   +8.62%     
==========================================
  Files         416      416              
  Lines       34096    34109      +13     
==========================================
+ Hits        29836    32788    +2952     
+ Misses       4260     1321    -2939     

Flag	Coverage Δ
unittests	96.13% <100.00%> (+8.62%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
.../p4est_2d_dgsem/elixir_navierstokes_convergence.jl	100.00% <100.00%> (+100.00%)	⬆️
.../p4est_3d_dgsem/elixir_navierstokes_convergence.jl	100.00% <100.00%> (+100.00%)	⬆️

... and 47 files with indirect coverage changes

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[ranocha]

Thanks!