Single path for multi- and single- GPU

[carterbox]

Purpose

Unify the multi- and single- GPU code paths to improve readability and maintainability.

Approach

Removes additional if-else statements for single- and multi- GPU implementations by using function substitution and the Pool class.

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

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

[carterbox]

The num_gpu parameter is still in the argument list of recon(), but internal functions can access the attributes of the Pool to get the number of GPUs. If you are implementing some kind of primary and secondary groupings of GPUs, then this can be included as a property of the pool. Is that acceptable?

[xiaodong-yu]

Yes, that's acceptable. It would be great to add GPU grouping as a feature of the pool.