Confusing design of Scenario and single_run

[TomMonks]

The Scenario class allows a user to set a random_number_set. This is also settable with single_run. In effect single_run overwrites Scenario. The result can be confusing. for example

random number set is set via Scenario, but the model is run in single_run mode without setting a seed. This means that the model runs with a random seed as single_run used None.

The design needs to be modified to set the seed in one place.