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.
Merge pull request #1051 from rwcarlsen/prev-decay

fixed prev_decay_time tracking bug. better comments.