timeit magic: Implement '-o' ("once") switch and minutes/hour formatting

Implement a "-o' switch in timeit magic, which runs the cell/line once ("-r0 -n0") and better formatting for longer times (minutes/hours).

Currently minutes and hours use simple float formatting, not "hours:minutes:seconds" (1.5 min is 1 minute 30 secs).

fixes: 2751