Change to use SFTP server

[adriaat]

• Have you added unit tests for the new functionality?
• Have all tests passed in your local clone?
• Have you written documentation that is easy to understand?
• Have you added an example?
• Has the formatting of your code been checked?

Addresses #115. I only tested that we can download files with the upcoming change from ICARE. Finding files by day did not work with the previous code (maybe I was doing something wrong?!). I did not bother to fix more as I am aware of the ongoing refactoring.

Also, I had to install beautifulsoup4 in a clean environment to add the identity and download a test file.