make sure NEURON simulation runs are isolated

[anilbey]

When NEURON simulation runs are not isolated, the results can get corrupted and memory not freed - leading to a larger memory usage.

This happened in BlueBrain/BluePyEModel#111 when BPO processes were not isolated in tests using the --forked parameter. The tests failed as the simulation results were different. BPO should already guarantee that isolation. The systems depending on BPO cannot know it otherwise.

cc @AurelienJaquier @GianlucaFicarelli