Support setting the OMP_NUM_THREADS environment variable.

[JohnHalleyGotway]

Describe the New Feature

MET issue dtcenter/MET#1926 and corresponding PR dtcenter/MET#1977 adds an initial level of OpenMP support to MET for the met-10.1.0_beta5 development release. The initial implementation applies only to the computation of the fractional coverage fields computed by Ensemble-Stat and Gen-Ens-Prod. However, this support will be expanded in future versions.

The OMP_NUM_THREADS environment variable defines the number of threads to be used by OpenMP. If left unset, a single thread is used by default. Recommend adding a new METplus configuration option to define the number of threads the used. The functionality for this is very simple. Just take the setting from the [config] section and use it set the OMP_NUM_THREADS environment variable prior to running the MET tools.

One question is what name to use for the [config] section entry.

• Calling it OMP_NUM_THREADS would be very clear and consistent with the environment variable. However, differentiating between the OMP_NUM_THREADS config option and the environment variable of the same name could cause confusion.
• Calling it METPLUS_NUM_THREADS or MET_NUM_THREADS would avoid that confusion, but perhaps cause other types of confusion?

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[georgemccabe]

In the default METplus config file, there is a variable called TMP_DIR which sets the MET_TMP_DIR environment variable. If MET_TMP_DIR is already set in the user's environment, then that value will be used instead. We would likely want to implement the same behavior for this variable. I don't have a strong preference on the name of the METplus config variable, but MET_OMP_NUM_THREADS was my first thought. The variable could be set to 1 in the default config file so it could be adjusted for a given installation or overridden by the user.