Bug fixes, portability enhancements, and small functionality enhancements

• Fixed several generic warnings (thanks michel-slm!)
• cpuRelax added for PowerPC and ARM (thanks barracuda156!)
• Added missing header (thanks ryandesign!)
• Try to detect and add libatomic when required (thanks for discussions barracuda156!)
• Enable small buffers from small buffer allocators to go down to 4 bytes (thanks for discussion David Caruso!). This is handy for 32-bit builds where pointers are typically 4 bytes
• Ensure that NOMINMAX is propagated for CMake Windows builds (thanks SeaOtocinclus!)
• Fix some cases using std::make_shared for types requiring large alignment, which is a bug prior to C++17 (thanks for help finding these SeaOtocinclus!)
• Set up CI on GitHub Actions, including builds for Mac and Windows in addition to Linux (thanks SeaOtocinclus!)
• Add an envinronment variable DISPENSO_MAX_THREADS_PER_POOL to limit max number of threads available to any thread pool. In the spirit of OMP_NUM_THREADS. (thanks Yong-Chull Jang!)
• Slight change of behavior w.r.t. use of maxThreads option in ForEachOptions and ParForOptions to limit concurrency the same way in both blocking and non-blocking for_each and parallel_for (thanks Arnie Yuan!)
• Various fixes to enable CMake builds on various 32-bit platforms (thanks for discussions barracuda156!)
• Updates to README