Allow MATSim XML config as well

[paulheinr]

We are currently relying on our own yml config format. Especially when we are going to implement simple scoring and replanning we need to think about the config again:

• Do we really want to have two seperate config dialects? MATSims XML config dialect is already relatively powerful
• Do we need to change to Config struct therefor?

To separate concerns (as we did it for all other input files) with serde, we would need to structs: One only for parsing serde, the other is the internal config data structure.