export segmentation with layers from different groups

[jessdtate]

right now, when a user exports a segmentation with layers from different groups, the exporter doesn't complain and tries to do it, but is often wrong. The exporter still assumes that they are part of the same group so it just changes the data values in one of the volumes that actually correspond to those in the other. This means that if the volumes are different sizes (dimensions), the layer from one of the groups will be sampled incorrectly and the values will be scattered. if the dimensions are the same it can work, but the origin/spacing information will be ignored from one of the groups.

It seems that a warning would be useful in informing the user that using layers from different groups may yield garbage, or to disallow it altogether.

[allywarner]

#287

[allywarner]

Add an error or warning when trying to export from different groups.