Concatenates all data into one matrix, performs SVD, keeps r
principal components, then computes a naive PPCA logLik and BIC.
Examples
if (FALSE) { # \dontrun{
sim <- simulate_mixture_data(N = 50, K = 3, r = 2, M = 6, T_each = 100)
pca_res <- fit_single_pca(sim$list_of_data, r = 2)
print(pca_res$BIC)
} # }