Relocate SimulationNode for mobility models

[kilianvolmer]

Motivation / Current Behaviour

Currently metapopulation_mobility_stochastic.h includes metapopulation_mobility_instant.h because both use the same SimulationNode.

Enhancement description

The SimulationNode could be moved to a different location s.t. the two different mobility models have no dependencies on each other.

Additional context

No response

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project