Investigate Fortran warnings in `run_part.F90` and `run_part_opt.F90` (Type mismatch in argument 'this')

[Griger5]

Compiling with all the warning flags re-enabled (#476) results in these warnings:

  [886/888] /usr/bin/gfortran -I/home/graca/Desktop/code/PyPartMC/src -I/tmp/tmp6kafbq7p/build/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/json/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/nanobind/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/nanobind_json/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/span/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/string_view-standalone/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/optional/include -I/usr/include/python3.12 -O3 -DNDEBUG -O3 -fPIC -fimplicit-none -ffree-line-length-none -Wno-surprising -fno-stack-protector -Wall -Wextra -Wpedantic -Werror -Wno-unused-parameter -ffunction-sections -fdata-sections -Wno-error -fpreprocessed -c CMakeFiles/_PyPartMC.dir/src/run_part.F90-pp.f90 -o CMakeFiles/_PyPartMC.dir/src/run_part.F90.o
  /home/graca/Desktop/code/PyPartMC/src/run_part.F90:10:6-6:

     10 |   use pmc_run_part
        |      1
  Warning: Type mismatch in argument 'this' (CLASS(sub_model_data_t)/CHARACTER(1)) between (1) and (2)
  [887/888] /usr/bin/gfortran -I/home/graca/Desktop/code/PyPartMC/src -I/tmp/tmp6kafbq7p/build/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/json/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/nanobind/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/nanobind_json/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/span/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/string_view-standalone/include -I/home/graca/Desktop/code/PyPartMC/gitmodules/optional/include -I/usr/include/python3.12 -O3 -DNDEBUG -O3 -fPIC -fimplicit-none -ffree-line-length-none -Wno-surprising -fno-stack-protector -Wall -Wextra -Wpedantic -Werror -Wno-unused-parameter -ffunction-sections -fdata-sections -Wno-error -fpreprocessed -c CMakeFiles/_PyPartMC.dir/src/run_part_opt.F90-pp.f90 -o CMakeFiles/_PyPartMC.dir/src/run_part_opt.F90.o
  /home/graca/Desktop/code/PyPartMC/src/run_part_opt.F90:10:8-8:

     10 |     use pmc_run_part
        |        1
  Warning: Type mismatch in argument 'this' (CLASS(sub_model_data_t)/CHARACTER(1)) between (1) and (2)

Due to -Werror they result in errors. They can be silenced in CMake but disabling -Werror for those two files:

set_source_files_properties(
  src/run_part_opt.F90
  PROPERTIES COMPILE_OPTIONS "-Wno-error"
)
set_source_files_properties(
  src/run_part.F90
  PROPERTIES COMPILE_OPTIONS "-Wno-error"
)