Hang when program finishes if running parallelised

[johnomotani]

When parallel execution is enabled, hypnotoad often (always?) hangs when the program finishes, and needs to be terminated with ^C.

It seems there must be somehow a circular reference somewhere, maybe between Mesh and its ParallelMap member variable?

Edit: Actually the __del__() methods do (at least sometimes) get called after a keyboard interrupt. Maybe that suggests the hang is somewhere else? Unless the interrupt somehow breaks some circular reference chain and lets the garbage collection gets started??

Note: this 'hang' happens after grid generation is finished and the grid file is closed, so it should not cause any errors.