Classification in subTOM

There are several classification strategies within subTOM. Three are variants of Multivariate Statistical Analysis (MSA), and the final is Multireference alignment. Within all of these there are some functions which are generally applicable and so they are shared between each modus of classification. This includes the clustering of MSA data (subtom_cluster), as well as the averaging of data that has been classified (subtom_parallel_sums_cls and subtom_weighted_average_cls). Since classification, particularly prinicipal component analysis (PCA), is very operation intensive there is also a function to pre-align all extracted particles and write them to disk to speed up later processing steps (subtom_parallel_prealign).