API: Purge difference map from tike.ptycho

[carterbox]

DM is not implemented correctly, and we have better algorithms anyways, so removing it.

Purpose

Approach

Pre-Merge Checklists

Submitter

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

Reviewer

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

[pep8speaks]

Hello @carterbox! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file src/tike/ptycho/ptycho.py:

Line 544:81: E501 line too long (88 > 80 characters)