Fix implementation of ellipse bounding boxes for feature vectors

[mzur]

The bounding box of ellipses used to generate feature vectors is currently not generated correctly (here). Here is how ellipses are represented in BIIGLE:

The bounding box (red) will not cover the entire ellipse.

Thinking about it, maybe this is an issue with rectangles, too. Check how rectangles are stored (the four points are the corners or the middle of the lines, like the ellipse). Rectangles are fine.

References #128

[mzur]

Last time we checked, ellipses were almost never used. Maybe we can even live with this compromise. What is the worst case of a mismatching bounding box here?