Support Alternate Path for ngen Logs

[mxkpp]

This adds support for an alternate path for ngen logs by persisting OS env var NGEN_RESULTS_DIR if already set.

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This was tested in conjunction with the following PRs of other repositories:

For essential functionality changes:

NGWPC/nwm-cal-mgr#34

https://github.com/NGWPC/nwm-rte/pull/44

For pending EWTS integration:

NGWPC/ngen-forcing#121

And this branch of ngen which also includes updates to various extern submodules: https://github.com/NGWPC/ngen/tree/cmaynard_ewts_separate_repo

[cmaynard-ngwpc]

This is an elegant solution because the forecast manager simply sets the environment variable for the ewts ngen logging use and do not actually use it. They maintain their own internal reference for accessing the results directory. For them, it is simply a set it and forget it scenario.

[kyle-larkin]

Achieves the objectives - facilitating non-ngenCERF loglevel setting, and allowing logfiles to be written to alternate paths.