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 would just leave the constructor signatures alone and don't add any new constructors.  Just add:

```c++
decaymode = "manual";
```

to the existing constructors.  If anybody wants anything different, they have to set the member directly:

```c++
SimInfo myinfo(/*blablabla*/);
myinfo.decaymode = "notmanual";
```