modify region growing 3D to work with low seed point value

Summary:
The 3D region growing tool did not work correctly for a low seed point value. The problem was caused by two parts in the code.
First, in the QmitkAdaptiveRegionGrowingToolGUI.cpp there was always 1 subtracted from the lower threshold, which caused the value in the regionGrower to be set to a high number if the lower threshold is already the lowest intensity in the image.
This is fixed by only subtracting/ adding 1 to the threshold bounds if this still results in values that are within the image pixel values.
Second, the itkAdaptiveThresholdIterator stopped as soon as the seed point value was not greater than the minimal threshold. As the seed point value is equal to the minimal threshold for the lowest value in the image, this condition was adapted.
These change fixes T28437.

Test Plan: manually tested

Reviewers: kislinsk, O1 MITK Reviewer Group I, floca

Reviewed By: O1 MITK Reviewer Group I, floca

Subscribers: floca

Maniphest Tasks: T28437

Differential Revision: https://phabricator.mitk.org/D486