Add support to set figure title in plot method

[toufiqmusah]

Description

Included plt.suptitle() to the plot_volume() method in visualization.py, to enable plotting subjects with a main title.
Added two args (figtitle: str None, fig title_y:int 0.75).

Here is a GitHub Gist of it working, along with tests.

No title:

Title:

Checklist

• I have read the CONTRIBUTING docs and have a developer setup ready
• Changes are
  • Non-breaking (would not break existing functionality)
  • Breaking (would cause existing functionality to change)
• Tests added or modified to cover the changes
• In-line docstrings updated
• Documentation updated
• This pull request is ready to be reviewed

[fepegar]

Thanks, @toufiqmusah.

[fepegar]

Thanks for your contribution, @toufiqmusah.

@allcontributors please add @toufiqmusah for code

[fepegar]

@allcontributors please add @toufiqmusah for code

[allcontributors]

@fepegar

I've put up a pull request to add @toufiqmusah! 🎉

[toufiqmusah]

@fepegar Thanks for the merge.

Though I will have to address the issue of the y offset. You were right to callout the hardcoded value probably affecting examples like FPG. The current implementation only works will when viewing subjects with two rows ore more (like FPG).

I will work on a fix in the coming days and make an update.