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'm +0 for end-of-table column addition as non breaking, pending @yarden-livnat or @hayagur 's confirmation that this doesn't break existing cyclus/cyclist2 functionality. It's my understanding that this will break HDF5's backwards compatibility whether or not its a column or table, right? That is, if a column, the table's `field_types` is incompatible, and if a table, it will not exist pre 1.1?