add detailed description for class2tree

[trvinh]

Description

Add a detailed description for class2tree to explain how the function works

#' @details See [vegan::taxa2dist()]. Thanks to Jari Oksanen for
#' making the taxa2dist function and pointing it out, and Clarke & Warwick
#' (1998, 2001), which taxa2dist was based on.
#' The taxonomy tree created is not only based on the clustering of the taxonomy
#' ranks (e.g. strain, species, genus, ...), but it also utilizes the actual 
#' taxon clades (e.g. mammals, plants or reptiles, etc.). The process of this 
#' function is as following: First, all possible taxonomy ranks and their 
#' corresponding IDs for each given taxon will be collected from the input.
#' Then, the rank vectors of all taxa will be aligned, so that they together 
#' will become a matrix with columns are a list of ordered taxonomy ranks of all
#' given taxa and rows are the rank vectors of those taxa. After that, the rank
#' matrix will be converted into ID matrix, any missing rank will have a pseudo
#' ID from the previous rank. Finally, this ID matrix will be used to cluster
#' taxa that have similar taxonomy string together.

Related Issue

#849

Example

[sckott]

thanks! some questions

[trvinh]

@sckott can you please check again? I have changed all the points from your comments. I hope that it is ok now.