Additional image loading options

[ofgulban]

Some load options can be useful, such as:

• [x] load a gradient magnitude image to bypass its calculation.
• [ ] load a previous segmentation (acquired via other tools) to be manipulated by adding/subtracting segmentator's labels.

[ofgulban]

--gramag flag is added to load gradient magnitude image.

[ofgulban]

load a previous segmentation (acquired via other tools)

I think this one can wait for the next version. I am removing this from v1.1.0 milestones.