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.
uncomment LoadTableTypes()
empty blob constructor
Merge pull request #1060 from scopatz/empty-blob

empty blob constructor
Merge pull request #1059 from scopatz/h5fix

uncomment LoadTableTypes() in HDF5 backend
Merge pull request #1051 from rwcarlsen/prev-decay

fixed prev_decay_time tracking bug. better comments.
lazy decay option and misc cleanups and tweaks

* lazy decay option decays materials every time the Material::comp function is
  called.

* Added ability to call Material::Decay with a negative time argument that
  causes the decay operation to run with the material's context's current
  time.  Throws if the material has a NULL context.

* fixed sim_init tests - they previously counted on optional normalization for
  material compositions that has now changed.  Note that API is identical -
  the doc comments for the relevant functions explicitly states that returned
  composition are not normalized.