Simplify calls to State methods from Scenario object

[danielolsen]

Currently, any calls to methods of the Scenario's state attribute (an instance of a State child class) need to be used as e.g. scenario.state.create_scenario(), scenario.state.launch_simulation(). It would be nice if the user didn't have to think about the state, and could simply call scenario.create_scenario() or scenario.launch_simulation(), while all of the same underlying logic is maintained (i.e., calling a method for a state that the Scenario is not in still gives an error).