Add date validation and coercion utilities

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Summary

Add utility functions for validating and coercing date inputs.

Functions

• ngr_chk_coerce_date() - Coerce various date-like inputs to Date class with validation
• ngr_chk_dt_complete() - Check if a date range is complete (no missing dates in sequence)

Use Case

Validating date inputs for hydrometric functions and ensuring complete time series data.