NEW: Implement ePIE for ptychography

[carterbox]

Purpose

Implement the extended ptychographic iterative engine (ePIE) algorithm based on Maiden's 2009 paper.

Approach

Started with the lstsq_grad implementation because they are similar, but changed the update directions and gradients accordingly. There are a few significant differences between the paper and this implementation:

1. This implementation allows batches of more than one pattern to be processed simultaneously. In this case, the object/probe updates are averaged across all the positions in the batch.
2. This implementation allows for multi-probe illumination. In this case, the wavefront updates are based on the gradients of the Gaussian noise model as implemented in the other methods. Then the probes are updated in a simultaneous mode i.e. each mode uses the same object patches to compute its own update and the object updates from all probes are averaged.

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] Write tests for new functions or explain why they are not needed.
• [ ] Build the documentation successfully
• [x] Use yapf to format python code.

Reviewer

• [x] Actually read all of the code.
• [x] Run the new code yourself.
• [x] Write a summary of the changes as you understand them.
• [x] Thank the submitter.