change num_ to n_ in ts.pca?

[petrelharp]

As discussed in #3008, elsewhere in tskit we do n_X; but in ts.pca we have num_oversamples and num_components. The latter matches scikit-learn, but I'm voting for internal consistency in tskit.

[jeromekelleher]

+1 for internal consistency. We can add synonyms later for compatibility with other libraries if its useful

[petrelharp]

And, I got this backwards. Elsewhere in tskit we do num_X.