Put extra features in SpotType.

SpotType can now have extra features.
Still many things to do:
1. Put privileged features (X, Y, Z, R, Q, Frame) setters in ImgSpot.
2. Deal with features that are of long type and not of double type.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>