NEW: Simple Multi-grid reconstruction method

Purpose

Add a user-friendly wrapper around ptycho.reconstruct that implements a simple multi-grid reconstruction.

Approach

Downsamples the ptychography problem by powers of 2 a requested number of times. Then performs the requested number of iterations at all levels. Each level provides the initial guess for the next higher-resolution level via upsampling. Parameters are upsampled using scipy.ndimage.zoom which uses spline interpolation. Used zoom because it was convenient, but also the splines provide smooth transition from low resolution to high.

Comparison of different re-gridding methods here: https://anl.box.com/s/uwbh80n0vz37dmtjbbnsig53m53as0fr

Pre-Merge Checklists

Submitter

[x] Write a helpfully descriptive pull request title.
[x] Organize changes into logically grouped commits with descriptive commit messages.
[x] Document all new functions.
[x] Click 'details' on the readthedocs check to view the updated docs.
[x] Write tests for new functions or explain why they are not needed.
[x] Address any complaints from pep8speaks.

Reviewer

[x] Actually read all of the code.
[x] Run the new code yourself; the included tests should make this easy.
[x] Write a summary of the changes as you understand them.
[x] Thank the submitter.

AdvancedPhotonSource / tike