trivial change to avoid taking a square root. should be slightly faster
added waterfill algorithm, but only added it for a single CPU for the moment
added "byhand" options for linear algebra, which assumes 2x2 matricies and performs the algebra by hand instead of delegating to numpy.linalg. This provides a significant speedup and makes posterior.set_P tractable for large n_pix without multi-processing, which was limitted by communication speed
added "byhand" option for hPrior.set_covariance that assumes 2x2 matricies and performs algebra by hand. This provides a significant speedup.
added "draw_noise" methods for the PSD and Network objects so they can simulat gaussian noise conveniently
added significant testing for "byhand" options and model_selection.waterfill
a reasonably large number of changes, mostly to allow zero_data and zero_noise realizations for testing