Allow CLI use without ecFlow installed

[elcarpenterNOAA]

Synopsis

• Detect whether the ecFlow API is importable, only add the ecflow mode when it is available, and lazily import uwtools.api.ecflow from ecFlow-specific functions.
• Add an informative ImportError when uwtools.ecflow is imported without the library available.

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.
• Where helpful, I have written comments in this PR's Files changed view to assist reviewers.