Improve segmentation module

[smistad]

• [ ] After a contour is drawn, convert it into a spline (e.g. cardinal spline), by creating regularly spaced control points along the contour.
• [ ] Make this feature optional, maybe put into separate module (e.g. spline module)
• [ ] If the contour is closed, create a closed spline. How to detect this?
• [ ] Allow user to edit the spline by moving the control points
• [ ] On save: convert spline back to an actual segmentation? Or store spline in database, convert to segmentation on export.
• [ ] On load: convert segmentation back to spline?