#' Add New Product to MODIS Collections
#' 
#' @description 
#' \code{addCollection} is a non-exported helper function to add a new product 
#' column to the product collections managed by \strong{MODIS} (see 
#' \code{MODIS:::collections}). Once added, the specified product will be 
#' tracked and, if required, kept up-to-date by \code{\link{getCollection}}.
#' 
#' @param product Character. Name of the product that should be added to the 
#' 'collections' dataset, see \code{\link{getCollection}}. 
#' @param collection Numeric. Optional information about available collections. 
#' If not supplied, this defaults to 'NA' and the user is required to manually 
#' retrieve information about available collections via 
#' \code{getCollection(..., forceCheck = TRUE)}. Note that the latter operation 
#' requires the previous execution of \code{MODIS:::addProduct} and 
#' \code{MODIS:::addServer} to make the newly added product available to 
#' \code{\link{getCollection}}.
#' @param path_ext Character. Path to folder containing file 
#' 'MODIS_Products.RData'. When working with RStudio projects (.Rproj), this 
#' usually defaults to 'inst/external'.
#' @param overwrite Logical. If \code{TRUE}, the initial '.RData' file located 
#' in 'path_ext' will be overwritten.
#' @param ... Currently not used.
#' 
#' @return 
#' A 'data.frame' which, for each product featured by \strong{MODIS}, holds 
#' information about available collections.
#' 
#' @seealso 
#' \code{MODIS:::collections}.
#' 
#' @author 
#' Florian Detsch
#' 
#' @examples 
#' \dontrun{
#' ## E.g., add collection of MODIS evapotranspiration product
#' MODIS:::addCollection(product = "MOD16A2", collection = 105)
#' }
#' 
# #' @export addCollection                     
#' @name addCollection
addCollection <- function(product, collection = NA, 
                          path_ext = "inst/external", overwrite = FALSE, ...) {
  
  ## load list of current products
  load(paste0(path_ext, "/collections.RData"))
  
  ## id of last and new entry
  int_id_last <- ncol(MODIScollection)
  int_id_new <- int_id_last + 1
  
  ## if missing, append NA entries to 'collection'
  int_len <- length(collection)
  if (int_len < nrow(MODIScollection))
    collection[(int_len+1):nrow(MODIScollection)] <- NA
  
  ## add new collection
  MODIScollection <- cbind(MODIScollection, collection)
  names(MODIScollection)[int_id_new] <- product

  ## output storage
  if (overwrite) {
    file_out <- paste0(path_ext, "/collections.RData")
    save(MODIScollection, file = file_out)
  }
  
  ## return updated collections dataset
  return(MODIScollection)
}