Fix create predictor collection

[adelavega]

Predictors were not being generated correctly (looping was done wrong, and nans were not skipped) This should fix that.

[codecov-io]

Codecov Report

Merging #628 into master will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   82.33%   82.31%   -0.03%     
==========================================
  Files          56       57       +1     
  Lines        2174     2177       +3     
==========================================
+ Hits         1790     1792       +2     
- Misses        384      385       +1

Impacted Files	Coverage Δ
celery_worker/app.py	0% <ø> (ø)	⬆️
neuroscout/resources/analysis/reports.py	63.41% <ø> (ø)	⬆️
celery_worker/tasks.py	0% <0%> (ø)
neuroscout/core.py	67.74% <100%> (ø)	⬆️
neuroscout/tasks/upload.py	55.71% <64.28%> (+1.3%)	⬆️

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