a bit of clean up and documentation for CellIter
start restructuring of CellListPotential
Start to refactor CellIter

primarily so coords does not need to be passed to the constructor
CellIter no longer needs coords in the constructor
add a test for LJCutCellLists

compare it to simple LJCut.  It fails.  There might be a bug in
CellLists
Fix a bug in CellIter

get_minimum_corner_distance2() was not implemented correctly.
add more tests for CellLists potentials

Also, print some warning messages in CellIter if the parameters seem
inefficient
rewrite CellIter::build_atom_neighbors_list

The new algorithm should be a lot simpler and avoid looping through
duplicate pairs.  I haven't tested whether it is actually faster.
clean up CellIter a bit
change warning message in CellIter from stderr to stdout
add some more documentation to CellIter
Merge pull request #98 from js850/cell_lists

refactor CellIter and CellListPotential