weather: Add functions for temperature in Fahrenheit

[FintasticMan]

This adds some functions to the WeatherService class for getting the temperature in Fahrenheit. This could theoretically overflow, but only if the temperature is 164.27 °C or higher (or -164.28 °C or lower), which I believe is unlikely enough that we don't need to worry about it. Please tell me if you think that is an issue.

This doesn't add a setting to display the temperature in Fahrenheit in PTS.

Related to #1783.

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	377496B	-16B
data	940B	0B
bss	63420B	0B

[Avamander]

I really wouldn't add the helper to the data struct definitions, that file is ideally just about the data.

There are other helper functions in the weather controller, that seems like a better place for it?

[FintasticMan]

Ah OK, I was just looking at what you said here, but I've just pushed a change that adds the functions to the WeatherService instead.

[Avamander]

Looks good in this form. I'd appreciate if for example @kieranc could test it out with PTS, if he has the time.

[matoro]

Hi, wondering if there's anything missing to allow this and the PTS change to be merged? Was very surprised to discover weather service does not support Fahrenheit.

[FintasticMan]

I don't think there's anything blocking this in particular. There are a couple of issues to be worked out with the weather implementation, which take priority over problems like Fahrenheit not being supported.

[FintasticMan]

This is superseded by #1805.