data_override: Data table schema

[J-Lentz]

Description A JSON schema for YAML data tables has been added at data_override/docs/data_table.json.

How Has This Been Tested? Works with Tom's YAML validation script and the land_xml data table.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules
• [x] New check tests, if applicable, are included
• [x] make distcheck passes