REF: Actually copy parameters / Update convergence metric

[carterbox]

Purpose

Actually return a copy of the parameters classes when using the copy_to_device/copy_to_host functions. This prevents unexpected behavior when a parameter of one of these classes is modified in place.

Use a convergence metric that checks how much (absolutely) the object has changed since the last epoch.

Approach

Return a new instance of the Parameter classes from the copy_to functions instead of a reference to the original.

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.