Oxidize units

In the current version I decided to use chrono::Duration and chrono::NaiveDateTime for time related fields as I started with the PrrtClock and there it seemed most fitting as it can get the current time and supports negative durations.

For the rest of the units I use however uom. The three options for time are:

• std::Duration,
• chrono::Duration,
• uom::Time.

This issue serves mostly the purpose of reminding me that I have to think about this again, but feel free to give your opinion.