[mlpack] PCA has to negate every element

[Shaoxiang Chen]

Because the matrices in mlpack are column-major, the results of pca is the
negative of correct results according to this line
<https://github.com/mlpack/mlpack/blob/21b198b20beae6e567dc8c1e47293ddde562bef2/src/mlpack/tests/pca_test.cpp#L76>
of code.
Isn't it more reasonable to negate inside Apply()?
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