Provide a sleep function in modbat.dsl.Model

[cyrille-artho]

Currently, stay allows a model to stay dormant for a while. The new code (issue #33) will integrate this functionality with an implementation that supports virtual time. This allows us to skip ahead to the next event, instead of waiting with Thread.sleep.
If we use this in a simulated environment, the environment also needs to be able to skip time. We therefore need to provide a custom sleep function in the model.
If virtual time is used, Model.sleep internally calls advance on the virtual timer instead of sleeping.
Later on, we can also support real time, where all calls to advance are replaced with Thread.sleep.