Add more informative Suggest dep messages

[bschilder]

For example, check_genome_build just runs requireNamespace. This may not be enough information for all users to indicate that they need to install this package.

[bschilder]

New internal function now facilitates this throughout EpiCompare:

check_dep <- function(dep){
  if(!requireNamespace(dep, quietly = TRUE)){
    stp <- paste("Package",shQuote(dep),
                 "must be installed to use this function.")
    stop(stp,
         call. = FALSE)
  }
}