Clean up and itemize return elements

[Neves-P]

Quoting @richelbilderbeek in #90:

#' The subsequent elements of the list each contain information on a single
#' colonist lineage on the island and has 4 components:\cr
#' \code{$branching_times} - island age and stem age of the population/species
#' in the case of Non-endemic, Non-endemic_MaxAge and Endemic anagenetic
#' species. For cladogenetic species these should be island age and branching
#' times of the radiation including the stem age of the radiation.\cr
#' \code{$stac} - the status of the colonist \cr * Non_endemic_MaxAge: 1 \cr *
#' ndemic: 2 \cr * Endemic&Non_Endemic: 3 \cr * Non_endemic: 4 \cr
#' \code{$missing_species} - number of island species that were not sampled for
#' particular clade (only applicable for endemic clades) \cr \code{$type_1or2}
#' - whether the colonist belongs to type 1 or type 2 \cr

This is a terrible return type to read. The return type is a list,
so I'd prefer the elements of the list to be itemized (using itemize).