update c++ example potential

There were several changes made to the c++ backed that were not
reflected in the example
add a test for LJCutAtomList

this tests the AtomListPotential, which is an alternate to
SimplePairwisePotential
add test for BLJ

this also tests CombinedPotential
add analytical hessian to CombinedPotential and AtomListPotential
rename tests to comply with nosetests

nosetest tests discovery won't recognize tests in files named
_test_object.py

also, nosetests with run a test twice if it imported somewhere else,
e.g. if it was imported in __init__.py.
Merge pull request #33 from js850/fix_nosetests

fix a few bugs with nosetests
AtomListPotential now uses shared pointers
Merge branch 'master' of https://github.com/pele-python/pele into periodic2D
implemented template distances

written distance as a template for both periodic and cartesian distance. These can now be used for arbitrary dimensions. The 2D and 3D case have a specialized template to avoid any performance loss.