add some start message

.onLoad <- function(...) {

  for(g in c("cor_md_tbl", "correlate", "mantel_tbl", "pro_tbl",

             "easycorrelation", "rcorr", "corr.test")) {

    register_s3_method("igraph", "as.igraph", g)

    register_s3_method("tidygraph", "as_tbl_graph", g)

  }

  invisible()

.onAttach <- function(...) {

  if (interactive() && ("ggcor" %in% loadedNamespaces())) {

    msg <- paste0("The `ggcor` package has been loaded, and it's some functions may conflict\n",

                  "with `linkET`, please detach or uninstall the `ggcor` package first.")

    packageStartupMessage(msg)

  }

register_s3_method <- function (pkg, generic, class, fun = NULL)

{

  stopifnot(is.character(pkg), length(pkg) == 1)

  stopifnot(is.character(generic), length(generic) == 1)

  stopifnot(is.character(class), length(class) == 1)

  if (is.null(fun)) {

    fun <- get(paste0(generic, ".", class), envir = parent.frame())

  if (!("ggcor" %in% loadedNamespaces()) &&

      ("ggcor" %in% row.names(installed.packages()))) {

    if (stats::runif(1) > 0.5) {

      msg <- paste0("`linkET` has almost all of `ggcor`'s features and includes\n",

                    "many new features, so switching to `linkET` is recommended.")

      packageStartupMessage(msg)

    }

  else {

    stopifnot(is.function(fun))

  }

  if (pkg %in% loadedNamespaces()) {

    registerS3method(generic, class, fun, envir = asNamespace(pkg))

  }

  setHook(packageEvent(pkg, "onLoad"), function(...) {

    registerS3method(generic, class, fun, envir = asNamespace(pkg))

  })

}