Hs wca interface

This adds one common interface to access many combinations of
cartesian / periodic
with / without frozen particles
with / without cell lists
do you think use_frozen is necessary? Isn't it implicit that if you do not pass a list of frozen atoms you don't want to use that option? Likewise for boxvec, if you don't pass a boxvec or boxl then the problem is unbounded by definition. I think we only need use_cell_lists.