meta cartesian distance policy
meta periodic distance policy
add cpp test, use std inner product, order includes
change dot to use begin() and end() of array
change sum function to use std::accumulate
change product to use std functions
change meta helper name to meta_periodic_distance
add documentation for meta_image, add references for round
add some more documentation to CellIter
formatting
Merge pull request #98 from js850/cell_lists

refactor CellIter and CellListPotential
Merge remote-tracking branch 'origin/master' into meta_dist
Merge remote-tracking branch 'smcantab_fork/copy_hs_wca_cell_test' into copy_hs_wca_cell_test
fix typos