add a c++ test for LJNeighborList
LJNeighborListTest uses class framwork
LJCut c++ test now uses class interface
a few minor changes to LJ tests
move PotentialTest to new file
morse potential uses new c++ class test
Merge branch 'master' of github.com:pele-python/pele
Get OTP rigid body working again

and simplify it down to the simplest case
remove a very old playground directory
remove the obsolete directory
Merge branch 'master' of github.com:pele-python/pele
Fixed confusing error message and lowered Cython requirement.
Merge pull request #31 from olenz/master

Fixed confusing error message and lowered Cython requirement.
Merge branch 'master' of https://github.com/pele-python/pele into array_operators