Logging and parallelization

[quaquel]

The three evaluators for parallelization (multiprocessing, ipyparallel, mpi) all have a way of sending log messages from the workers to the main process. This is convenient because it centralizes the log messages and ensures that errors appear. However, it is also overhead and potentially error-prone, most notably with MPI.

It might be convenient to have a simple flag that can be used to control whether to centralize the logging or not.