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. 

Decay MUST be added to the db or it will be impossible to run any
simulation with anything other than the default decay mode.  *All*
initialization for every simulation occurs from the db only and from
nowhere else.  I think @scopatz is suggesting we add a new table instead of
appending a column.


On Tue, Sep 2, 2014 at 4:04 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)
>
> Ok. So, neither a new column nor a new table seems to be a backwards
> compatible option. Is the current request that I just do it without adding
> the decay item to the db at all?
>
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> Reply to this email directly or view it on GitHub
> <https://github.com/cyclus/cyclus/pull/995/files#r17015739>.
>