Global decay off

This implements an optional flag that allows the user to disable the material class' Decay function. This is intended to fix issue #984 which complains of an inability to globally turn off decay (at the very least)

As we discussed before on a development call, this feature simply adds the option to allow or specifically not allow the use of the Decay function in the material class.

Further granularity of options or implementation of global decay was not supposed to be part of this quick fix. In the future, this work can be extended to re-implement the feature in which decay occurs whenever a material composition is queried. However, that implementation of global decay was supposed be left up to further discussion on what folks decide was the best course of action.

This PR focuses on adding the decay flag to the SimInfo object and using that flag in the material class' decay function. 

@gidden - Gotcha.

Ok, if there's really no way around it, I will add a new table just for this variable. 

That said, I think we can all agree that it is super ugly to have a table with only one member. So, I'll just note here that this implementation should be changed as soon as we allow ourselves to break the API again. This is another situation where it would be good to have a "2.0" milestone tag full of these niggling issues that ought be changed to the reasonable solution as soon as it's possible to do so.