Deleting data transform can remove visualization modules as well

[jtschwar]

Describe the bug A clear and concise description of what the bug is.

If the pipeline has a single data transform or operator (i.e. Mark data as tilt series or Gaussian Blur) and is deleted, all the visualization modules will be deleted with it.

To Reproduce Steps to reproduce the behavior:

1. Load Dataset
2. Click on 'Mark Data As Tilt Series'
3. Delete 'Mark Data As Tilt Series'
4. See error All the visualization modules (Slice, Outline, Etc.) will be deleted with it.

Expected behavior A clear and concise description of what you expected to happen.

It would be nice not having to reload these modules when a data operator is deleted.

Screenshots If applicable, add screenshots to help explain your problem.

Before:

I'll only delete 'Set Tilt Angles' but the rest will be removed with it.

Desktop (please complete the following information):

• Operating system: macOS 10.11
• Tomviz version: [e.g. 1.6.0-151]

Hardware (please complete the following information):

• Device: Macbook Pro