Show weather forecasts in Fahrenheit or celsius, depending on user's settings

[aaronbrethorst]

The map's displayed temperature unit should be determined by the user's system settings. The current value can be found as described in this SO answer: https://stackoverflow.com/questions/39727075/determine-users-temperature-unit-setting-on-ios-10-celsius-fahrenheit

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I expect that you'll end up with a property on ForecastManager.swift: public var formattedCurrentTemperature: String?.

Verification:

Let’s say that the current temperature is 68ºF.

• formattedCurrentTemperature should == "20º" if the user’s locale specifies celsius.
• formattedCurrentTemperature should == "68º" if the user’s locale specifies fahrenheit.
• If the current forecast is unavailable, then the formattedCurrentTemperature should be nil