Point generation: Give a choice to allow duplicate points or not

[StephenChan]

Branched off from issue #308.

'Duplicate points' means points with the same row, column in the same image. Our random point-generation algorithm up to now has generated points without regard to preventing duplicates, i.e. it uses sampling with replacement.

We're not sure if a significant amount of researchers would specifically want no duplicates, i.e. sampling without replacement. If so, we could offer a choice in the point generation algorithm: perhaps an 'Allow duplicate point locations' checkbox next to the point generation method fields in New/Edit Source.

How disallowing duplicates would work: After generating one point location, if it's a duplicate of a point location we generated previously for this image, try generating that one point again. Retry a maximum of N times (perhaps N = 10). If we cannot get a unique point location after N tries, then cancel the process that initiated point generation (image upload, annotation area reset, etc.) and display an appropriate error message.

Considerations:

• CPCe allows duplicate points.
• Regardless of what CoralNet's point generation does, people can still take the trouble to generate their own points outside of CoralNet and then import them via CSV/CPC.

This issue currently has no milestone, because we're not sure if people would find this useful.