use chunked mappers in the threadpool, make sure that the map function actually waits for all the threads to have started.

Commented out, note gparts test is unchecked.

When moving foreign particles to the end of the list, make sure we don't swap with one that is a foreign particle.

Added a command-line option '-d' to execute a dry run which read the parameter file and opens the ICs to check whether everything is OK.