Modern diag manager: subregional corner diagnostics fix

[uramirez8707]

Description

• Adds logic to prevent diag manager from duplicating subregional data at the edges.
• Adds test to test subregional corner diagnostics and checks the answers
• Fixes issue that was causing the edges data to no be written

Fixes # (issue)

How Has This Been Tested? CI, including new tests

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