Login code cleanup

[alejandrox1]

Is your feature request related to a problem? Please describe.
The majority of scripts will check if a user already has an access token or create it for the user.
The code that does this is repeated throughout the place. i.e.,

agave-cli/bin/files-cp

Lines 49 to 60 in db55abb

	agave = Agave()
	config_file = "{}/config.json".format(cache_dir)
	# Check if there is a session already saved.
	if os.path.exists(config_file):
	agave.load_configs(cache_dir=cache_dir)
	else:
	print("You need to initiate a session first. Use auth-sessions-init")
	sys.exit(1)
	# Refresh tokens if necessary.
	agave.refresh_tokens()

Describe the solution you'd like
This could all very well be organized into a function, login(), that all other scripts can import.