Correct possible misspelling
FeatureStackArray: do not set reference index until feature updating.

Otherwise the first slice of the input stack would be used as reference and it can be a problem if no samples are traced on it. Bug reported by Douglas K. Benn.
In the GUI, set slice to the same slice as the input.
Increase version number also in the GUI.
Make release 2.1.3

Major changes:
* Fix bug that was causing exception when using a 3d image as input when no traces where set in the first slice.
* Set slice in the GUI so it is the same as the one selected in the input image.