Pushed new branch T22403-ImageStatisticsAreAlwaysComputedTwice.

Bug is not in the plugin (oops).
The plugin calls the GetStatistics method twice, but should not trigger the calculation 2 but instead simply return the already computed statistics at the second call.
The bug is caused by improper handling of modified() functionality in the image statistics calculator and in the mask generators. Whenever the time step is changed, both will be modified which is noticed by the image statistics calculator and will lead to recomputation of statistics.