add read_with_horizon method

[sahiljhawar]

resolves #12

[Copilot]

Pull Request Overview

This PR adds the read_with_horizon method to ensemble classes for reading forecast data at specific forecast horizons. The method enables filtering ensemble data based on the lead time from forecast creation to target time.

• Adds read_with_horizon method to Kp and Hp ensemble classes for horizon-based data reading
• Refactors file listing logic into separate helper methods for better code organization
• Updates test suites with comprehensive coverage for the new horizon functionality

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
swvo/io/kp/ensemble.py	Implements read_with_horizon method and refactors file listing into ensemble_file_list helper
swvo/io/hp/ensemble.py	Adds abstract read_with_horizon method to base class and concrete implementations for Hp30/Hp60
tests/io/kp/test_kp_ensemble.py	Adds comprehensive test cases for Kp ensemble horizon functionality
tests/io/hp/test_hp_ensemble.py	Adds test coverage for Hp ensemble horizon methods with validation and error cases
tests/io/dst/test_wdc.py	Updates test date to avoid potential data availability issues

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