Download a file with error checking

dod.download() is mainly intended for use by other functions in the dod package.

Usage

dod.download(
  url = NULL,
  file = NULL,
  destdir = ".",
  age = 0,
  silent = TRUE,
  debug = 0
)

url: character value giving the web address of a file to be downloaded. This has different values for different file types.
file: character value giving the name to be used for the downloaded file.
destdir: a character value indicating the directory in which to store downloaded files.
age: a numerical value indicating a time interval, in days. If the file to be downloaded from the server already exists locally, and was created less than age days in the past, it will not be downloaded again. Setting age=0 (which is the default) forces a download.
silent: logical value passed to download.file(), which does the downloading. The default, TRUE, indicates not to show a progress indicator.
debug: an integer value indicating the level of debugging. If this exceeds 0, then some debugging messages will be printed. This value is passed down to related functions, but with 1 subtracted for each pass.

dod.download returns a character value holding the full name of the file, including the path to destdir.