Change format of the event output file

[leo-desbureaux-tellae]

Problem exposition

At the current date, the output file containing the simulation events is stored in .txt file, with events formatted as strings using their str() method.

This makes the event file little usable from outside, as it would demand a very specific parsing.

However, the event log of the simulation can be very useful, as it contains the rawest description of the whole simulation.
For instance, it can be used to compute KPIs instead of having to compute them in Starling, or generate visualisation files using a different format from Starling.

Proposed solution

Generate the event output file at a more common format for this type of file.

The best format for this seems to be XML (it is the format used by MATSim for its event log)