VOTable distances

[Carifio24]

This PR adds support for properly using the distance/altitude coordinate to VoTableLayer so that these layers will display at the correct position in the sky in Solar System mode - currently, VOTable layers display their points at a radius of 1 AU from the Sun (see #178). Since SpreadSheetLayers already support this, this is accomplished largely by mimicking the corresponding code inside that class. The relevant changes are in the PrepVertexBuffer method of VoTableLayer.

Related to this, when the distance column is set in the VOTableViewer, currently the unit is assumed to be light years. This PR changes that, using the unit for the relevant VoColumn (which is already parsed into the VoTable). To do this, I added a method in UiTools that grabs the appropriate AltUnits value based on the abbreviation given as the unit. Please let me know if there are any different/additional string representations that this should support.

One other thing worth mentioning:

• The code for this operation in SpreadSheetLayer used its method GetScaleFactor. As it was, this method had no real reason to be attached to SpreadSheetLayer - although not marked as static, it doesn't use any of the instance's member values. Since the calculation for VoTableLayer would require the same code, and this method was almost entirely identical to UiTools.GetMeters, I moved this function into UiTools and refactored GetMeters to use GetScaleFactor.

[pkgw]

Awesome! @astrodavid10, do you know how to navigate to the Azure Pipelines artifacts to download an installer for the bleeding-edge version of the app that includes this fix?

[astrodavid10]

@pkgw just figured it out! I'll test it out.