fixed prev_decay_time tracking bug. better comments.

* extracted materials now inherit prev_decay_time properly from originating
  material.  Test for this was added.

* when being absorbed, use prev_decay_time of the material with the larger
  quantity.

* added prev_decay_time public accessor to facilitate testing and other
  potential uses by archetypes.
fixed

On Wed, Dec 3, 2014 at 2:19 PM, Matthew Gidden <notifications@github.com>
wrote:

> In src/material.h:
>
> > @@ -128,12 +128,20 @@ class Material: public Resource {
> >    /// a reactor.
> >    void Transmute(Composition::Ptr c);
> >
> > -  /// A special case of Transmute where the new composition is calculated by
> > -  /// performing a decay calculation on the material's composition.  The time
> > -  /// delta is calculated as the difference between curr_time and the last time
> > -  /// Decay was invoked.
> > +  /// Updates the material's composition by performing a decay calculation.
> > +  /// This is a special case of Transmute where the new composition is
> > +  /// calculated automatically.  The time delta is calculated as the difference
> > +  /// between curr_time and the last time Decay was invoked.  This may or may
> > +  /// not result in an updated material composition.  If the simulation decay
> > +  /// mode is set to "never" or none of the nuclides' decay constants are
> > +  /// significant with respect to the time delta.
>
> only a conditional clause
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/cyclus/cyclus/pull/1051/files#r21261604>.
>