Adding null objects to the scenario

[sammacbeth]

We should check against null objects being added to the scenario and throw a useful exception. Currently the exception raised - when the null object is injected - does not lead back to the source of the error:

Exception in thread "main" java.lang.NullPointerException
    at com.google.inject.internal.InjectorImpl.injectMembers(InjectorImpl.java:943)
    at uk.ac.imperial.presage2.core.simulator.RunnableSimulation.initialise(RunnableSimulation.java:301)
    at uk.ac.imperial.presage2.core.simulator.RunnableSimulation.run(RunnableSimulation.java:271)
    at uk.ac.imperial.presage2.core.simulator.RunnableSimulation.main(RunnableSimulation.java:164)