Dataframe subsetting fixes#2

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emmaSkarstein wants to merge 2 commits intooharar:masterfrom

emmaSkarstein:dataframe-subsetting-fixes

Commits on Mar 11, 2020

Changed all cases of df[,column-name] to df[[column-name]] in order to avoid problems when subsetting tibbles (without changing behavior for regular data frames).
emmaSkarstein
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Commits on Mar 18, 2020

Changed back to original subsetting some places.
emmaSkarstein
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