Add an option in python runner to execute an arbitrary script after fork

Another new feature to facilitate debugging and profiling. Allow to execute an arbitrary script in the python runner just after forking. The script may contain a format string {pid} which will be replaced with the PID of the corresponding process. There are three options to execute: for the mother process, for the first fork, or for each fork in a sequence. The script process is started in the background and its outputs are redirected to files.

I plan to map this option to athenaHLT.py command line argument --scriptAfterFork, mirroring the option provided in this MR for runHLTMPPy.py.

Potential use cases:

# profile memory in one fork
athenaHLT.py --scriptAfterFork "firstFork:heaptrack -p {pid}" <other-args>

# run vtune collection for one fork
athenaHLT.py --scriptAfterFork "firstFork:vtune -collect hotspots -target-pid {pid}" <other-args>

# run perf collection for all forks
athenaHLT.py --scriptAfterFork "allForks:perf record -o perf_{pid}.data -p {pid}" <other-args>

Plus anything you can imagine.

@fwinkl, @wiedenma, @ssottoco, @smh please let me know if you would like this feature to be merged.