Trainable Weka Segmentation: add input error control.
Trainable Weka Segmentation: when creating training instances from a shape roi, use in-place method.
Trainable Weka Segmentation: clean code to try to identify bug that is causing huge delay when adding instances whose attributes are all 0.0.
Trainable Weka Segmentation: add warning in javadoc to limit the use of getProcessor in FeatureStack.
Trainable Weka Segmentation: remove calls to getProcessor() in the FeatureStack for more reliable methods.

Maybe the ImageStack field of FeatureStack should be substituted by an array of ImageProcessor to prevent heavy duplications of the data.
Merge branch 'master' into trainable-seg-gui