Use .get() lookups for optional schism, ww3 template_value items

[maddenp-cu]

Synopsis

The template_values item under namelist for schism and ww3 is optional per those drivers' schemas (and based on user feedback, this is a good thing, in that they might like to provide a complete, ready-to-use file via the template_file value and not apply any Jinja2 evaluation to it), but access in the driver code is via [] lookup, making them de facto required. Change [] lookups to .get() with a {} default value.

Type

• Bug fix (corrects a known issue)

Impact

• 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.