VelocityAnalysis: move feature declaration and calculation to thresholder.

To save memory by avoiding transferring data between classes.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
VelocityAnalyser: fix bug with track shorter than the smoothing window.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
VelocityAnalyser: better place the dialog.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
VelocityAnalysis: make the new feature keys public.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
VelocityAnalyzer: partial data export.

Missing average track intensity and co.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
VelocityAnalysis: add track intensity statistics.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>