Create Overview Scan panel from WideFieldAcquisition.py

[lizziewylie]

Fixes #69

[CLAassistant]

All committers have signed the CLA.

[cmeyer]

Minor drive-by comments. I'm aware this is draft, btw.

[cmeyer]

I'm only adding review for coding style, not for functionality or general algorithm logic.

[Brow71189]

I haven't tried it, just a comment from reading through the code inline below. Apart from that it looks fine.

[MattRoyle]

Mostly pedantic. Main bits are suggesting that the exporting is done via existing export functionality rather than introducing a new library dependency PIL. The UI line edit handling is done on a per function basis, converting to integers, exiting early if the value is too large. I think using Converters in tandem with property getters and setters would make the code more readable.

[MattRoyle]

Some issues with the defocus logic and how wide this panel appears.

[MattRoyle]

If this path already exists then this will override the previous file. It might be useful to allow the flexibility of a UI to name the file with it defaulting to overview-scan with an index incremented to avoid duplicate names overview-scan(1). This way you can switch back to a previous scan without rerunning. In order to properly achieve this you would also need to save the properties json to a file. Then provide a user interface for reloading previously taken overview scans. I think that would be useful but probably something for a follow up.