ensure plotting in check_assumptions (file mixins.py)

[nomennominatur]

The plotting code section for check_assumptions in mixins.py has been moved so that it will plot Schoenfeld residuals whenever show_plots = True, regardless of the numeric test result.

Justification:

- Consistency of behaviour of check_assumptions

- Visual plausibility checking of PH test result.

(This is my first ever pull request, so please bear with me, if I did something wrong or inadvertently have violated the code of conduct. Feedback appreciated.)

[CamDavidsonPilon]

Yea, I like this: the user is being explicit about plots been shown, and so the computer should obey.

PR looks good!

• good description of problem
• good solution with reasons why it is a good solution

Thanks @nomennominatur!

[nomennominatur]

Thanks @CamDavidsonPilon for the fast review & merge, and also for the kind reception! I feel honored having made a miniscule contribution to such a wonderful project.