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. 

I think that a new table should be created rather than simply adding a column.  This line breaks backwards compatibility of the data base interface.  A db created with v1.0 would not be able to re-run or restarted with v1.1. Furthermore, any tools that are built to read and interface with v1.0 or v1.1 would experience an extra column or lack thereof. For maximum utility, I believe that the tables as published in http://fuelcycle.org/user/dbdoc.html should stand. Modifications to these tables should come in the form of new tables, not new columns.  This is especially true in the decay case where this column may be removed or deprecated in the future.