Fix broken covariate/phenotype validation

[karenfeng]

What changes are proposed in this pull request?

Switches to pandas-style verification.

How is this patch tested?

• Unit tests
• Integration tests
• Manual tests

This broke the linreg test in the GWAS notebook I was running as part of a release; this fix resolved the issue.

[codecov]

Codecov Report

Merging #343 (21d2539) into master (54db807) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   93.69%   93.69%           
=======================================
  Files          95       95           
  Lines        4823     4823           
  Branches      473      473           
=======================================
  Hits         4519     4519           
  Misses        304      304           

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