Vantor ISD

Add support for Vantor (formerly Maxar, formerly DigitalGlobe) ISD delivered as an XML file.

Issue originated in https://github.com/conda-forge/usgscsm-feedstock/issues/44 with input from @acpaquette
> Ideally, we won't have to create new drivers for the format but add something to another existing script (either isd_generate or usgscsm_cam_test) that would convert the Vantor XML into an ISD. If you want, you can also make an issue on USGSCSM for an enhancement to read the Vantor XML directly.

USGSCSM issue: https://github.com/DOI-USGS/usgscsm/issues/524

Sample Level 1B Vantor data through EarthData for NASA funded researchers
https://search.earthdata.nasa.gov/search?q=Vantor

The IARPA WRIVA program has released some Maxar (now Vantor) imagery
https://ieee-dataport.org/open-access/wriva-public-data

ASP has code to parse the XML ISD file
https://github.com/NeoGeographyToolkit/StereoPipeline/blob/6778e85af5692871cd2492a7e2433c4a7f793aa6/src/asp/Camera/LinescanDGModel.cc#L51

Start by examining the existing isd_generate and usgscsm_cam_test entry points, then compare their handling with the ASP LinescanDGModel.cc parser and the linked USGSCSM issue. Use the linked Vantor sample sources to determine the XML fields and choose whether conversion or direct reading fits. Done means Vantor XML ISD data is supported without a new format-specific driver.

Written by the indexing model from the issue text.