Otf2

This branch adds a basic support for the OTF2 trace format. The main interesting events are supported (function entry/exit, MPI P2P communication, metrics) and I was able to open medium-sized traces (up to 500MB) on my laptop (for larger traces, you'll need lots of RAM, or a few optimizations in the code).

The code is stable enough and ViTE is usable on most OTF2 traces, so I think it's time to merge it into the master branch.