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
