Split/rescope paper

[aimalz]

The paper is now overgrown and needs to be pruned, but the three points it makes are all valuable. This issue can be closed when @davidwhogg is satisfied with the three smaller papers into which I will split the current version. For reference, the papers will be scoped as follows:

• Paper 1: A thorough presentation of the CHIPPR model and chippr prototype code, which are currently Sec. 2 and App. A, demonstrated on the three canonical forms of photo-z systematic error (bias, scatter, catastrophic outliers), which are currently Secs. 3.1, 3.2, 3.3.
• Paper 2: A deeper exploration of the implicit prior and model misspecification, which is currently Secs. 3.4 and 4.2.
• Paper 3: The propagation of CHIPPR results to cosmological parameter constraints, which is currently Sec. 4.1.

I wonder if there should also be another paper about the forward model of photo-z PDFs, which I've been meaning to separate out into a standalone package anyway. . .