Option to "restart" annotation, use existing points layers

[sofroniewn]

A complement to #9

[jni]

This is great, and could be "easily" implemented by adding reference_pts: Optional[layers.Points], moving_pts: Optional[layers.Points] to the start_affinder function signature. If they are not set, we create them, but if they are set, we use the existing ones.

[jni]

@tlambert03 I don't think there's optional support in magicgui yet, right? ie I'd like to have a combobox for existing points layers in the UI but there should be a "None/create" option in the combobox...

[tlambert03]

Nope not yet. Discussion happening here