Minor changes to Forcings Engine Data Provider for derived implementations

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This PR makes 2 changes to support derived implementations:

1. Moves timing and configuration file path member variables outside of private scope into protected so derived implementations can directly access these.
2. Adds ForcingsEngineDataProvider::ensure_variable, which is a helper function that helps handle variable name aliasing from what NGen expects to what the forcings engine expects.

Additions

• ForcingsEngineDataProvider::ensure_variable

Changes

• init_, time_begin_, time_end_, and time_step_ from private to protected scope.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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