Avoid 500 errors when jumping between steps

[paulapreuss]

Right now some steps should not be accessed before completing other steps, even though they are clickable

• Jumping to step 8 before a simulation has been conducted lead to an error, same if clicking on "results" from project list

[paulapreuss]

Related to this: When running a simulation and then going back and forth between steps there are often problems with the Business Model. The error business_model.models.BusinessModel.MultipleObjectsReturned: get() returned more than one BusinessModel -- it returned 2! comes up when simulating and then going back and forth between system setup and simulation