Minimise the number of times `roi` is called in `Refocus.autofocus`

[PinkShnack]

Problem

In most of my cases, if I am calling the Refocus.autofocus method, I am using the same value for the roi argument.

Therefore, every time I call this method, the roi checks are being performed even though they don't need to be.

Possible Solutions

Implement roi as a Refocus class attribute, which will be checked once when instantiating the class or anytime a Refocus.parse_roi() method is called (in the case that users want to change the roi each Refocus.autofocus call). See #16 and #17.

Notes

The Refocus.autofocus roi argument could be deprecated or left as is for convenience.

I would say this issue is low priority, because the speed gains are likely small.

To do:

• Implement roi as a Refocus class attribute
• Refactor the roi docstring a little bit so that it displays nicely with sphinx (use bullet points, similar to the metric docstring param description.