Fix incorrectly reported volume when using .5 as volume step

[henrikwidlund]

Fixes the volume displayed on the remote so that it isn't constantly .5 too high

[zehnm]

I have to discuss this first in our team. Technically, 0.5 is rounded up, but I'm not an "audio-head" and don't know what makes more sense in terms of volume level.
I'll also check if there are plans to show 0.5 volume levels in the UI.

If we are going forward, the Python code format has to be fixed so they pass the linter checks: docs/code_guidelines.md

[henrikwidlund]

I also have no idea what is more correct, but as a user it is confusing that it says it's louder that it is. Removing the need for rounding in the first place would be ideal, or allowing for specifying how the rounding should work on the remote as a global setting.

Happy holidays!