Feature #2742 develop fix StatAnalysis to set fcst_lev/obs_lev

[georgemccabe]

instead of fcst_level/obs_level to match actual MET config variable names. Added unit tests to ensure correct behavior occurs

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[JohnHalleyGotway]

This is the same approach for the develop branch that's used for the main_v5.1 bugfix. Keep all the config options the same in METplus, just replace instances of _level with _lev.

I did check the docs for FCST_LEVEL_LIST and OBS_LEVEL_LIST and confirmed that they correctly state that they're used to set FCST_LEV and OBS_LEV columns of the MET output. So the documentation is already correct.

I approve.