Feature 407 stratosphere bias plot

[CPKalb]

Pull Request Testing

• Describe testing already performed for these changes:
  
  Tested the METplus use cases to check that they produce the plots needed

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Check out the following branches of METplus, METcalcpy, METplotpy, and METdataio
  feature_1344_stratosphere_bias of METplus
  feature_342_write_mpr_file METcalcpy
  this branch of METplotpy
  bugfix_247_readnc_no_lon METdataio

Run 2 use cases, s2s/UserScript_fcstGFS_obsERA_StratosphereBias.conf and s2s/UserScript_fcstGFS_obsERA_StratospherePolar.conf. Verify that that the StratosphereBias use case produces 2 plots (GFS_ERA_ME_2018_02_zonal_mean_T.png, and GFS_ERA_ME_2018_02_zonal_mean_U.png), and the StratospherePolar use case produces 4 plots (ME_2018_02_polar_cap_T.png, ME_2018_02_polar_vortex_U.png, RMSE_2018_02_polar_cap_T.png, and RMSE_2018_02_polar_vortex_U.png).

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]
  Yes

• Do these changes include sufficient testing updates? [Yes or No]
  ??

• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  
  No

• Please complete this pull request review by [Fill in date].
  
  Soon.

Pull Request Checklist

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• Add any new Python packages to the METplus Components Python Requirements table.
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[bikegeek]

successfully generated the 2 plots from the UserScript_fcstGFS_obsERA_StratosphereBias and the four plots from UserScript_fcstGFS_obsERA_StratospherePolar