Configuration default timestep (and other values?) is not working

[mmcleod89]

• If no timestep is given in config file, the program will fail rather than run using the default
• if there is an year of intervention but not an intervention option then a similar problem occurs

There is some issue with the experiment class trying to unpack these values and assuming that they are there. (They also cannot be blank.)

The simulation constructor has defaults but the experiment class which reads the yaml does not.