Use gdown instead of downloader

[achalddave]

This adds a dependency on gdown, but I found it much more stable than using the downloader.py used here. With downloader.py, I constantly received the error noted in the readme, despite repeated attempts ("Maybe you need to change permission over 'Anyone with the link'?")

[sylee-skhu]

Does this commit solve the problem("Maybe you need to change permission over 'Anyone with the link'?")? and.. how about adding gdown to environment.yml?