BUG: Disable log scale on plot_cost_convergence when negative

[carterbox]

Purpose

Prevent error from plotting Poisson cost function objectives.

Approach

Disable the log scale on the y-axis if any of the cost function values are non-positive.

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.