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. 

Seems fine. My main hesitations with that were twofold :
1) strings are more annoying to input-check
2) calling it decay and allowing it to be turned "on" or "lazy" at this point is quite false and could mislead the user. Since all this flag actually can do is allow or disallow decay, at this point, I'm afraid calling it decay and allowing it to be turned on will signal to the user "decay is now on". So, it seemed I should make the distinction more clear. 

As a compromise, how about I throw a warning ("Global decay is not currently implemented in cyclus. Decay is merely allowed, not guaranteed.") if the user sets decay to anything other than "off". What do you think?