time(it) magic: format timespans up to hours in human readable form

* Refactor time formatting of timeit into a new function
* Add minutes and hours for long running cells/lines
* Convert time and timeit magic to use the new function

Minutes and hours are float formatted, so 1.5 h is "1 hour, 30 minutes".

Signed-off-by: Jan Schulz <jasc@gmx.net>