Improving DICOM import robustness

By trusting DICOM Tag for having spacing between slices, we had importing issues with some images (ask me if you want them). 
Thus, I changed the way we obtain spacing between slices before calculating sliceLocation.

So first, we fill the Dictionnary in with all the tags of the images (ReadHeader function).
Then, call DetermineSpacing
And finally calculate sliceLocations.

Feel free to test this PR with different DICOM images.