feat(): Added variable output units to the variable context.

[benflexcompute]

Note

Adds automatic persistence of output units for post-processing user variables.

• Enhance save_user_variables to derive output_units via batch_get_user_variable_units and persist them in variable_context (converting numbers to expression with dimensionless where applicable)
• New batch_get_user_variable_units to compute units from default_context values (handles Expression, unyt_array, and numbers)
• Reuse get_post_processing_variables to scope which variables get unit inference
• Update tests and reference JSON to include additional variables and assert output_units (e.g., km/ms, m/s, dimensionless)
• Remove obsolete commented code

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