Fixes segfault in LennardJonesFluid and LennardJonesGrid

Merge pull request #33 from jchodera/segfault-workaround

[WIP] Segfault workaround

0.6.1

Merge pull request #30 from kyleabeauchamp/py3

Fixes to init.py

0.6

Merge pull request #28 from jchodera/add-sobol

Added Sobol subrandom sampling approach to initialize particle positions