Passive Acquisition cannot load multiple images

[jepietryga]

Describe the bug
In using Passive Acquisition, there is an issue where the first file will load correctly, but the second will crash tomviz.

To Reproduce
Following the Passive Acquisition tutorial, I am able to set up the server and begin monitoring files. Next, I load a .tiff following the standard naming convention into the monitored folder--this tiff is loaded, though the slice is listed as the maximum negative value for integers. The next image dragged into the folder will then crash tomviz.

Expected behavior
The tiffs should be loaded into one datasource.

Desktop (please complete the following information):

• Operating system: Linux
• Tomviz version: 1.9.0

Hardware (please complete the following information):
Several different devices

Additional context
I believe the issue is with how tomviz Datasource's appendImage function works. It seems like it's only used by Passive Acquisition, and some reading into vtk indicates how images are appended might've been changed since Passive Acquisition was implemented. I tried to fix it, but I am not experienced enough in vtk to come up with a working solution. Maybe the vtkImageAppend class could be useful?