Create CPU/OMP analog of `setQuESTNumGpuThreadsPerBlock`

[TysonRayJones]

Create env-varQUEST_DEFAULT_NUM_CPU_THREADS

This allows users to control specifically QuEST's parallelisation, without affecting other softwares in the user stack. It also parallels QUEST_DEFAULT_NUM_GPU_THREADS_PER_BLOCK (after refactoring the current cmake/compile-time var to an env-var)

CreatesetQuESTNumCpuThreads().

This allows users to runtime override of the number of threads (useful for sweeps/profiling) without needing to link user code to OpenMP. This is relatively unimportant - a competent user performing tuning should be able to interface with OMP directly - but should be included for symmetry with the existing functions (default env-vars with runtime overridable functions)

[TysonRayJones]

(These facilities generally making performance profiling/tuning much easier for users or external contributors. We might learn of new thresholds! 🎉 )

[TysonRayJones]

If this is added, visit updating getQuESTEnvironmentString as discussed here

[TysonRayJones]

@otbrown Do you think this has any utility? It's mostly for symmetry with the new GPU threads stuff - the diff and API would be extremely similar to (but mercifully smaller than) 25dab1d (although I wouldn't bother with a CMake option). Could also take or leave the QUEST_DEFAULT_NUM_CPU_THREADS env-var.