Add support for the Statistical Weather Data API

[dblock]

Description

Adds support for OpenWeather's Statistical Weather Data API, a Pro-tier endpoint that returns statistically calculated (not real historical) weather data based on historical observations for a location.

• client.statistics_year(lat:, lon:) - stats for all 365 days of the year.
• client.statistics_month(month, lat:, lon:) - stats for a specific month.
• client.statistics_day(month, day, lat:, lon:) - stats for a specific day.

Adds OpenWeather::Models::Statistics::Aggregations (top-level response, Enumerable) and Aggregation (per-period result), plus Temp, Wind, Pressure, Humidity, Precipitation, Clouds sub-models. Adds history_endpoint config option, defaulting to https://history.openweathermap.org/data.

This is a paid/Pro-tier API, so specs use hand-crafted VCR cassettes built from OpenWeather's documented example responses (same approach as the existing Hourly/30 Day Forecast Pro specs).

Closes #43.

100% test coverage, 129 examples passing.

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[dblock]

I don't have a paid OpenWeather subscription, so the VCR cassettes for this Pro-tier API are hand-crafted from the documented example responses rather than recorded against the live API (same approach used for the existing Hourly/30 Day Forecast Pro specs). I won't be merging this myself without someone verifying it against a real paid account.