Create Stop Model API call

[AdrianDAlessandro]

Problem:
The models need to be initialised and run at the start of the simulation

Proposed Solution:
Yue's code will check the datahub for a certain value to determine when to stop the model. So this requires adding that value and endpoint to the datahub and changing that value from the vis system.

This can be combined with the Start button to create the logic for the restart button in the controller

Follows on from #41