Feature/pca

[kvtsang]

• More PCA stuff (e.g. 2nd axis, spread ...)
• Fix of memory leak of the original version
• New SparseTensor::fit_pca(bool store_spread, bool use_true_coord)

// spread along pca1 in voxel units (default)
// for true xyz coordinate (cm), use_true_corrd=true
// if spread is not needed, set store_spread=false for slightly better performance
auto pca = tensor.fit_pca(true);
pca.get_spread()[0];