Test with 2D and 3D data

[dstansby]

The widgets that take images should work with 2D and 3D data. This should be tested.

2D data:

• Histogram
• Slice
• Scatter

3D data:

• Histogram
• Slice
• Scatter

[dstansby]

@ruaridhg #113 is a start on this, but it doesn't close this issue yet. The idea is to test every widget with 2D and 3D data where possible. Perhaps you could make a checklist in the issue body above here with each widget and 2D/3D so we can keep track of where we've got to with this?