Easier loading of 2D data

[psavery]

We should allow the datasets in the HDF5 file to be 2D. In this case, we can just add another dimension. However, we should probably change a few things for 2D data:

1. We can just add another dimension of size 1 so it will work with the rest of the code
2. Opacity of all voxels should be 1, rather than the mean (we don't need opacity for a slice)
3. The VTK mapper doesn't accept data with a dimension of size 1. So actually, maybe we should add a dimension of size 2 and duplicate the data within each spot. Since it's just a slice, hopefully this won't be a big deal.