Rename repo

Created by: asogaard

As agreed at the weekly developer meeting on 7 December, we will rename the repo and the contained python package to "GraphNeT" (in plain text writing), short for Graph neural networks for neutrino telescope event reconstruction / graphnet (in code, file names, package name, etc.). This will require (1) changing the repo name in the settings and (2) changing names across code files. @philippeller, as the owner of the repo, is the only one who can do the first item; I will prepare a single, comprehensive PR for the second item.