Bulk download satellite data concurrently via asyncio

[weiji14]

Use python 3.5's built-in asyncio module to concurrently bulk download satellite data from http/ftp servers.

See: Hackernoon blog post asynctio aioftp docs aiohttp docs

[weiji14]

In order to be a bit nice on people's server infrastructure, and prevent FTP error 421 "Too many connections", use semaphores to limit number of simultaneous FTP connections.

Helpful examples of Python 3 implementations of Semaphores in asyncio:

• http://www.artificialworlds.net/blog/2017/06/27/adding-a-concurrency-limit-to-pythons-asyncio-as_completed/
• https://compiletoi.net/fast-scraping-in-python-with-asyncio/
• https://stackoverflow.com/questions/40836800/python-asyncio-semaphore-in-async-await-function#40836870

Official Python3 API docs on the implementation: https://docs.python.org/3/library/asyncio-sync.html#semaphores

Note use of async with sem: syntax in Python 3.6? But earlier versions (e.g. Python 3.4/3.5 ) may use something like with (yield from sem): or with (await sem).