A couple schema changes needed for GSI.

[christinaholtNOAA]

Synopsis

Allows GSI namelist to have empty sections, as required by GSI Fortran code.
Adds an optional stacksize entry to the execution: block.
Makes all files_copied, files_hardlinked, and files_linked tasks optional tasks when their sections are not included in the UW Driver Config, fixing a backwards compatibility bug.

Type

• Bug fix (corrects a known issue)
• Code maintenance (refactoring, etc. without behavior change)
• Documentation
• Enhancement (adds new functionality)
• Tooling (CI, code-quality, packaging, revision-control, etc.)

Impact

• This is a breaking change (changes existing functionality)
• This is a non-breaking change (existing functionality continues to work as expected)

Checklist

• I have added myself and any co-authors to the PR's Assignees list.
• I have reviewed the documentation and have made any updates necessitated by this change.