Add Poisson noise prior. Add bonus parameters <S> and <S^2> for and 2D.

* Add ability to set Gaussian prior on Poisson noise.  It might
  be worth changing this to a lower-bound prior instead.
* Add unit test for reading this in specfiles.
* Add <S^2> bonus param in 1D. (<S> already present from previous commit).
* Add <S> and <S^2> bonus params in 2D.