Add a simple way of resetting the accumulated timers

[gahjelle]

Today, it's possible to reset the accumulated timers by clearing the timers dictionary:

Timer.timers.clear()

However, this may lead to errors if a timer has already been initialized (typically because it's used as a decorator and initialized at import-time). Therefore a better way is to explicitly set existing timers to 0:

Timer.timers.update({k: 0 for k in Timer.timers})

However, this becomes hard to read, and even harder to write.

A better solution would be to have explicit support for resetting the accumulated timers.

[gahjelle]

Brainstorming:

• Have .timers be a UserDict-based custom class. This makes it easy to re-implement .clear().
• Store individual runs, not only total elapsed time. This allows us to do statistics on different runs
• Simple support for writing .timers data to disk? And reading back from disk?