[Feature] Make the cellfinder training data generation workflow clearer

[adamltyson]

I think it should be made clearer that the general workflow is:

• Run cellfinder
• Load curation widget
• Create new training data layers
• Add points to these new layers
• Extract cubes
• Train
• Run cellfinder
• Repeat curation/training as necessary, either by adding to the existing training dataset, or creating new ones

A user was curating the cellfinder results directly. This meant:

• Cube generation took forever
• Retraining the network didn't actually impact performance much