Returning ticks in CpuTime

[eminence]

Hello,
The time returned in procfs::CpuTime should be a number of ticks in u64 as it is for processes in procfs::process::Stat. It's better to be able to get the values as returned by the kernel.

Originally posted by @lparcq in #69 (comment)

[eminence]

I think this is a good idea, and wanted to open a dedicated to discuss it.

I do think that being able to get the time in seconds is useful. Maybe we could also provide some methods that would do the tick-to-second conversion

[lparcq]

Yes it would be convenient to provide a helper function to convert in seconds. However I would prefer a number of milliseconds as u64 than a floating number of seconds.