Add OneCall docu and add more tests

[edenhaus]

See #306 (comment)

@csparpa You can assign this issue to me. It is better to have an issue as to comment on a merged pull request. This week is already full with uni and work, but I think I can do it next week.

[csparpa]

@edenhaus I've written tests for weather_manager.one_call and weather_manager.once_call_history functions

Up to you writing tests for the OneCall class then

[edenhaus]

@csparpa nice :) I have already written some basic unit tests in the original PR. As OneCall uses the weather class, I think this enough tests for the moment. So I will concentrate on the docs next week.

When will be version 3.0 released?

[csparpa]

There is no release date but I would love to release v3 as soon as possible so that people can start using the OneCall feature as a swap-in replacement for the dead DarkSky API

No need to hurry!

[edenhaus]

Nice. I will then include the new version into home assistant as I will use oneCall to get the forecast for my smart irrigation