Some datasets have several empty groups 7fe0,0010 before a non empty group

[Paolo Brandoli]

Some dicom files have several groups 7fe0,0010 empty before a valid group 7fe0,0010 containing a valid image.

At the moment the dataSet::getImage function checks only the first datagroup and return null if it is empty

[Paolo Brandoli]

- Closes #70 Some datasets have several empty groups 7fe0,0010 before a non empty group

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