vine: API call to fetch temp files

[btovar]

A user may not know until runtime which files need to be transmitted back to the manager. For example, that decision may depend on some function evaluation, such as when to make a workflow snapshot, or which function call optimized some metric. In the case of the topEFT, the design simplifies if we mark all of the accumulation task results as temps, and then we simply fetch the file for the last accumulation.

These retrievals currently can be done by constructing a task, e.g.:

# generating the temp file
producer = Task("cmd > output.xyz")
my_tmp_output = declare_temp()
producer.add_output(my_tmp_output, "output.xyz")
m.submit(producer)
t = m.wait(...)
...

# fetching the temp file
fetcher = Task("ln -s output.xyz  output.to_fetch")
my_fetched_output = declare_file"my_final_output")

# using the yet to merge pr for fixed locations, as we don't want this file to be moved twice just for the transfer
fetcher.add_input(my_tmp_output,  "output.xyz",  fixed_location=True)
fetcher.add_output(my_fetched_output, "output.to_fetch")

fetcher.set_category("fetch_files")
fetcher.set_tag("my_final_output")
m.submit(fetcher)

t = m.wait(...)
if t and t.category == "fetch_files" and t.tag == "my_final_output":
   print("the file I wanted is now at the manager")

Thus, the API call to fetch files may simply construct a task as above.