Support nested Timers

[garth-wells]

It would be really handy if timers could nested. For example, a timer could time a function, and sub-timers could time sub-parts. Formatting could be nice with indentation.

One way to do this would be to have timers be able to create timers (sub-timers).

[chrisrichardson]

At the moment, the timers just register in the TimeLogger when they exit. It would be fairly easy to register when they start as well. In that way, it would be possible to see which timers are active together, and analyse it later.

What happens about a Timer in a loop, or which is called multiple times? Currently, these are aggregated.