Fix issue with OMP threading

[victorapm]

This pull request improves the threading mechanism in hypre, with a focus on optimizing thread utilization for functions where the workload varies based on the problem size. A specific example is the hypre_BoomerAMGBuildExtPIInterpHost function, which now uses an optimized approach for determining the number of threads when the number of rows is small.

Key changes:

• Added hypre_NumOptimalThreads for calculating the optimal number of threads based on the problem size.
• Updated OpenMP parallel directives to use num_threads, ensuring efficient thread allocation.
• Added detailed documentation for the threading utility functions.

Closes #1182