get child-project compute-durations to fail more gracefully when some durations are already present

[alecristia]

when durations are present, you get "duration exists, aborting"

We can make it more informative by providing options like:

A duration column already exists, so we are aborting the process. Run the same command with one of the following switches, depending on what you'd like to do:
-f: force compute duration for all files, regardless of previous values. This may be useful if you've edited your files somehow.
-a: add duration only for files with duration equal to zero