Calculate CPU times more accurately

[tombruijn]

This PR improves the reading CPU times from /proc/stat and the calculations we perform on them.

The issue

The CPU times weren't accurate as it didn't read all the values and didn't consider them in the calculations. Looking at htop we see a lot more work has to be done for user, nice and total times.

• htop: https://github.com/hishamhm/htop/blob/e0209da88faf3b390d71ff174065abd407abfdfd/ProcessList.c#L934-L955
• also read man proc, the /proc/stat subject

  Related issues

• Closes #22

[thijsc]

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