⚡ Bolt: Vectorize heat index calculation

[cmonteverde]

This PR optimizes the heat index calculation in nasa_data.py and era5_data.py by replacing row-wise df.apply with vectorized NumPy operations.

Changes:

• nasa_data.py: Vectorized calculate_heat_index logic using np.where for the conditional check (T < 26°C).
• era5_data.py: Vectorized calculate_heat_index logic, handling unit conversions (C to F and back) and conditional checks (T < 80°F) using NumPy.
• Added test_heat_index_performance.py: A new test file that mocks API responses and verifies the vectorized implementation produces correct results matching the original logic.
• Added .gitignore: To ensure build artifacts and cache files are not committed.

Performance:

• Benchmarks showed a speedup of ~114x for nasa_data.py logic and ~137x for era5_data.py logic on 100k rows.

Verification:

• Ran test_heat_index_performance.py (passed).
• Ran existing test_nasa_data.py (passed).

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PR created automatically by Jules for task 13939408731509633281 started by @cmonteverde

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