ENH: Adjustments and enhancements to `threading.h`

• Adjust the NoOfThreads class so that, if a value is not provided, or a value <=0 is provided, it will be set to a value read from the ${FSL_NUM_THREADS} environment variable, or 1 if that variable is not set.
• Add new parfor and apply functions for trivially parallelisable tasks. These functions perform essentially the same role as std::for_eaach and std::transform, but allow explicit control over the number of threads.