Misc issues to address

Nobody has claimed this yet.

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:

- [x] Update the build instructions in the top-level README.md file to enable OpenMP since we now use OpenMP in the exercises.
- [x] Distinguish between TOSS4 and blueos configurations for LC machines in README.md file and make sure builds work on both.
- [x] 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.
- [x] 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.
- [x] 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?

Start with the remaining unchecked items in the top-level README.md and CMakeLists.txt, then inspect the exercise files and current build instructions. Determine the required copyright updates and compiler coverage, and verify the documented local builds and supported configurations on the relevant LC systems.

Written by the indexing model from the issue text.