weather: Fix incorrect rounding for negative temperatures

[FintasticMan]

Thanks @tokiwee for reporting this issue!
Closes #2185 (if GitHub un-deletes it).

[github-actions]

Build size and comparison to main:

Section	Size	Difference
text	372848B	32B
data	948B	0B
bss	22536B	0B

[mark9064]

Would it be worth extracting out rounding integer division to some shared code? It's somewhat annoying that this isn't part of the C++ stdlib really :(

Maybe an implementation similar to this? https://github.com/lucianpls/Rounding-Integer-Division
I haven't tried it though

[FintasticMan]

What do you think of this? I've made a RoundedDiv function in the Utility namespace, that uses C++20 concepts so that we can have keep the code type-agnostic, while making sure that it only works for expected values.

Didn't you also use a rounded division somewhere in the AoD code? Would this solution also work there?

[mark9064]

Wow that looks great. I didn't know that concepts allow this, I should read up on it. Yes I think this would work for AOD, I guess that would go in a separate PR though?

I want to double check correctness properly before approving, but otherwise LGTM