Implement medinfo/common/ProcessManager.py

[jonc101]

Helper module to facilitate some recurrent process management tasks.
For example, support script currently to watch a process and wait until it finishes before spawning another process/command.
Many existing scripts that generate commands (with argv options) and then execute them serially. Develop option to do that while tracking separate log outputs, and another options to spawn a max number of child processes in parallel.