Add a model for near-field ptychography

[allevitan]

There's a near-field propagator, and given that the whole point of cdtools is to be flexible, it's kind of silly that there's no model for near field ptycho. Mael got 90% of the way there, so perhaps it is possible to build off of his work. Anyone interested in working on this can ask me for more info, or I will hopefully get to this soon.

[allevitan]

I started a draft pull request for this, #31

Anyone who is interested in this model can check out the progress on the associated branch