Check system timezone

[colinsauze]

The carbon intensity data we are given is in UTC. But system time could be in a different timezone. We need to translate this when producing the output from cats, currently we don't. We should also write unit tests to check this and to check what happens when the clocks change as this is a common way systems like this break!

[abhidg]

👍 https://xkcd.com/1883/ :)

[andreww]

Fixed by #61