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 appending columns to tables should be allowed as non-API breaking
and would be fine here (i.e. move the AddVal to the end of the sequence).


On Tue, Sep 2, 2014 at 12:05 PM, Katy Huff <notifications@github.com> wrote:

> In src/context.cc:
>
> > @@ -133,6 +136,7 @@ void Context::InitSim(SimInfo si) {
> >        ->AddVal("Handle", si.handle)
> >        ->AddVal("InitialYear", si.y0)
> >        ->AddVal("InitialMonth", si.m0)
> > +      ->AddVal("DecayAllowed", si.decay)
>
> Ah, yes, too bad. I am curious though why it breaks restart. I imagine it
> depends on how the queries are made. That is, when restart happens, does it
> memory map directly onto the database, or does it actually query by column
> name? If it queries by column name, then it shouldn't matter if there's an
> extra column. But, in the hdf5 backend, for example, you probably would
> just directly map onto the database structure. Is that what's going on?
>
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> <https://github.com/cyclus/cyclus/pull/995/files#r17000413>.
>