Copy a file in a directory. If root is set, copy also the
sub-directories. For instance, copy_in ~root:"/foo" "/foo/bar/gni"
"/toto" creates "/toto/bar/gni".
val download : overwrite:bool -> ?compress:bool ->t->Dir.t->t
download a remote file in a given directory. Return the location
of the downloaded file if the download is successful.
Compress activates http content compression if supported. May break
on gzipped files, only use for text files