0.7.0 prerelease

• Many bugfixes
• All integrators are now classes
• More integrators (rRESPA, etc.)
• All testsystems now have a topology property giving the OpenMM Topology object
• All testsystems now have mutable system, positions, and topology objects
• More testsystems (JAC DHFR benchmark, updated Src explicit/implicit)
• OpenMM platform energy and force comparison test script
• Options for shifted potentials for some systems

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