Add a new "-o" to timeit to run the cell/line only once

Such a parameter makes it easier to time longrunning cells/lines (as
used in data analyses).

Signed-off-by: Jan Schulz <jasc@gmx.net>
timeit magic: time formatting for minutes and hours

Use minutes and hours for long running cells/lines

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