adds Gradient worker test for non-uniform axes
fixes shape conversion in Gradient test
fixes wrong dimensions in Gradient test
fixes #96: ImageProcessing: Gradient worker does not respect axes

Axes discretization is now respected in the calculation of
the gradient magnitude.
Merge pull request #97 from aheld84/master

fixes #96: ImageProcessing: Gradient worker does not respect axes
Merge branch 'master' into manual
doc: finishes GUI part of the quick tour in the manual

replaces doc/manual/fig/gui_imagevis_gradient.png due to change in
Gradient worker (#96).