def update
      @data.each(&:update) if Daru.lazy_update
    end
Method for updating the metadata (i.e. missing value positions) of the
after assingment/deletion etc. are complete. This is provided so that
time is not wasted in creating the metadata for the vector each time
assignment/deletion of elements is done. Updating data this way is called
lazy loading. To set or unset lazy loading, see the .lazy_update= method.