Rewrite the online-reader with uproot4

A complete rewrite of the online.py is required to support the new uproot4 and awkward1 packages which were recently renamed to uproot and awkward so that those are the standard versions when pip install-ing them. As of km3io v0.19.1, the online.py uses the uproot3 package which was created for backwards-compatibility.

This is also a follow-up for #42 (closed)