Makefile requires environment variables
Created by: MPapendrecht
Currently the Makefile requires environment variables to be set for the username and password. I'd like to propose to make these variables optional. If they aren't set we fall back to the pip authentication instead of raising an error. This is also more inline with the netsquid Makefile. To make sure this is obvious to the user, the INSTALLATION.md could be updated to explain how the user can set these environment variables, or similarly to the netsquid Makefile add an echo statement.