Improvement: fix clang warnings

[dutkalex]

Describe your changes here:
This PR fixes a handful of warning, mostly related to casts during string interpolation.

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[dutkalex]

It seems like fixing the -Wunused-private-fields clang warning triggers a gcc -Wattributes warning, which causes the CI failures:

/usr/bin/c++ -DT8_CC=\"/usr/bin/cc\" -DT8_CFLAGS="\" -O1 -Wpedantic -Wall -Werror -Wextra\"" -DT8_CMAKE_BUILD -DT8_CXX=\"/usr/bin/c++\" -DT8_CXXFLAGS="\" -O1 -Wpedantic -Wall -Werror -Wextra\"" -DT8_ENABLE_DEBUG=1 -DT8_ENABLE_MPI=1 -DT8_ENABLE_MPIIO=1 -DT8_ENABLE_NETCDF=1 -DT8_LDFLAGS=\"\" -DT8_LIBS="\"NetCDF::NetCDF P4EST::P4EST SC::SC MPI::MPI_C\"" -DT8_PACKAGE_STRING="\"t8 4.0.0\"" -DT8_TEST_LEVEL_INT=1 -DT8_VERSION=\"4.0.0\" -DT8_VERSION_MAJOR=4 -DT8_VERSION_MINOR=0 -DT8_VERSION_PATCH=0 -DT8_VERSION_POINT=RC2-498-g53544fc80 -I/__w/t8code/t8code/src -isystem /__w/t8code/t8code/p4est/build/Debug/install/include -isystem /__w/t8code/t8code/sc/build/Debug/install/include -isystem /usr/include/x86_64-linux-gnu/mpich -isystem /__w/t8code/t8code/test/../thirdparty/googletest-mpi -isystem /__w/t8code/t8code/test/.. -O1 -pedantic -Wall -Werror -Wextra -flto=auto -ffat-lto-objects -std=gnu++20 -MD -MT test/CMakeFiles/t8_gtest_partition_data_parallel.dir/t8_forest/t8_gtest_partition_data.cxx.o -MF test/CMakeFiles/t8_gtest_partition_data_parallel.dir/t8_forest/t8_gtest_partition_data.cxx.o.d -o test/CMakeFiles/t8_gtest_partition_data_parallel.dir/t8_forest/t8_gtest_partition_data.cxx.o -c /__w/t8code/t8code/test/t8_forest/t8_gtest_partition_data.cxx
/__w/t8code/t8code/test/t8_forest/t8_gtest_partition_data.cxx:82:32: error: 'maybe_unused' attribute ignored [-Werror=attributes]
   82 |   [[maybe_unused]] t8_locidx_t a;
      |                                ^
/__w/t8code/t8code/test/t8_forest/t8_gtest_partition_data.cxx:83:25: error: 'maybe_unused' attribute ignored [-Werror=attributes]
   83 |   [[maybe_unused]] char b;
      |                         ^
cc1plus: all warnings being treated as errors

The obvious fix would be to simply delete the two fields, but maybe the test relies on the class having multiple fields?

[sandro-elsweijer]

Some minor stuff. Thanks for the contribution!

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.05%. Comparing base (4a06e41) to head (e43d6c5).

Files with missing lines	Patch %	Lines
src/t8_cmesh/t8_cmesh_io/t8_cmesh_readmshfile.cxx	60.00%	2 Missing ⚠️
...nnectivity/t8_cmesh_vertex_conn_vertex_to_tree.hxx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1982   +/-   ##
=======================================
  Coverage   77.05%   77.05%           
=======================================
  Files         112      112           
  Lines       18959    18959           
=======================================
  Hits        14608    14608           
  Misses       4351     4351           

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