API: Make psi parameter of tike.ptycho.reconstruct positional

[carterbox]

Purpose

Realized that the psi parameter is required, but has a default value of None. This is left over from a previous API breaking change where I removed auto-initialization of psi. This fix moves psi to the position parameters, so that the error raised tells the user that psi is required instead of that None has no parameters.

Also, adds a parameter to the velociprobe data loader, so that the data cropping can be controlled by the user.

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.
• [ ] Click 'details' on the readthedocs check to view the updated docs.
• [ ] Write tests for new functions or explain why they are not needed.
• [x] 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.

[carterbox]

Open to better ways to handle this API breaking change.