Initial conditions are required even when starting from snapshot

[JamesRunnalls]

The initial conditions file must exist and contain correctly formatted data even when starting from a snapshot.

[f-baerenbold]

Currently, the snapshot is read after the initialization of the model. So there are many things during the initialization that depend on the initial condition file. Changing this would require changes to the model that are a bit more time consuming. But this should probably be done at some point.