Export Tally Data to VTK capability

[pshriwise]

This PR adds functionality for exporting the current view of tally data to a VTK image file (.vti) for 3D visualization of tally results.

For tallies with mesh filters, the bounds and resolution of the VTK file is predetermined by the tool and cannot be altered by the user.

For spatial domain filters, the user will specify min/max bounds for x, y, and z as well as the resolution in each dimension for the image file.

If the vtk module is not installed, then this feature is disabled.

For large datasets (big meshes or a fine resolution specified by the userr), a progress dialog will appear so the user knows something is happening. Additionally, domain information (cell/material ids) can be exported along with the tally info.

Other updates:

• Image generation functions for tally data have been moved into the plotmodel file. That way all image generation (for cell/material ids, temp/density info, and tally data) all live in the same module.

• Selected filter bins, scores, and nuclides have now been added to the model so the dock doesn't need to query widgets for the selected values. This allows for better separation of the GUI info and model.

• Another set of changes from FM to font_metric.

• Updated format for global variables related to tally info (as suggested in Tally visualization #33)

[paulromano]

Export worked great for me. I checked with both mesh and cell filters and they behave as expected. One thing I was wondering about -- after I export the data, the dialog box still stays active. Is that intended? There doesn't seem to be any indication that the export was completely successfully.