set specific column order for covariates

[ginberg]

see #326

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.52%. Comparing base (253dd0f) to head (3c3853d).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
R/GetDefaultCovariates.R	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #329      +/-   ##
===========================================
- Coverage    84.57%   84.52%   -0.05%     
===========================================
  Files           16       16              
  Lines         1692     1693       +1     
===========================================
  Hits          1431     1431              
- Misses         261      262       +1     

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