Allow Java VM restart

[dg46]

Currently, the Java VM cannot be restarted after it is shut down. This limitation can be avoided if we run all tasks that require the VM in a separate process using the multiprocessing module: https://github.com/LeeKamentsky/python-javabridge/issues/88 This would also allow us to run many causal discovery procedures in parallel. A downside is that the start of a new process adds additional overhead that will affect the runtime of an analysis.