FrangiVesselness: Auto-fill the spacing field

It is silly to have to fill the _spacing_ field when running [FrangiVesselness.java](https://github.com/imagej/imagej-ops/blob/master/src/main/java/net/imagej/ops/commands/filter/FrangiVesselness.java), as 99.9% of the time folks would want to run using the _actual_ voxel size of the image. The field should be initialized before the prompt is displayed to the voxel dimensions of the image metadata

Start in src/main/java/net/imagej/ops/commands/filter/FrangiVesselness.java and inspect how the spacing field is prompted and how image metadata provides voxel dimensions. The change is done when the field is initialized from the image's actual voxel size before the prompt appears; check the command's existing behavior afterward.

Written by the indexing model from the issue text.