Misc issues to address

[rhornung67]

I think we want folks to be able to clone the tutorial repo and build locally without running in a container (e.g., on LC)

I ran into a few issues when trying to build locally on LC machines. Addressing the following issues will help:

• Update the build instructions in the top-level README.md file to enable OpenMP since we now use OpenMP in the exercises.
• Distinguish between TOSS4 and blueos configurations for LC machines in README.md file and make sure builds work on both.
  • For TOSS4, change module load gcc/8.3.1 to module load gcc/10.3.1 since that is the system default, and gcc 8.3.1 is longer available.
• Add set (RAJA_ENABLE_EXERCISES Off CACHE BOOL "") to the top-level CMakeLists.txt file since RAJA does not have a CMake dependent option for this and setting ENABLE_EXERCISES to Off builds all the RAJA exercises and drops the executable for each in the tutorial bin directory, which is confusing.
• Add Copyright information to files.
• Move solution files in exercises up one level and remove solution subdirectories. Configure solution files to build when code is built.
• Try building with clang and maybe some other compilers and versions and include info about what works in the README file?