NEW: Implement Alignment operator as composition with correct adjoints

Purpose

Provides true forward and adjoint operations for the alignment problem.

Approach

Apparently, the adjoint operator for a linear flow/interpolation operation is a scatter of the interpolated points using the same weights from the gather operation [1]. We are doing a very similar operation in the USFFT just with a different interpolation kernel.

[1] http://www.reproducibility.org/RSF/book/gee/ajt/paper_html/node10.html

The rotation operation is just interpolation.

The padding operation adjoint is the unpadding operation if and only if the padding values are zero.

While the true adjoints exist for padding with zeros, I have also added the ability to choose a non-zero constant value for interpolation and padding because the background of our ptychography reconstructions is non-zero.

Pre-Merge Checklists

Submitter

[x] Write a helpfully descriptive pull request title.
[ ] Organize changes into logically grouped commits with descriptive commit messages.
[ ] 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

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

AdvancedPhotonSource / tike