Allow Subsample for RasterPCA about use

[danddr]

Dear ailich, I see your point, but when we created the USE package we aimed to uniformly subsample the environmental space and this require fitting PCA on all the pixels and not only to a subset. I fear introducing the feature you are suggesting might create confusion among the users and misuse of the USE package.

[ailich]

@danddr, that is reasonable. Also, just a note but PCA is now directly implemented in terra rspatial/terra#1361

[danddr]

Noted! thanks!