Cell lists with cartesian distance policy

This addresses some issues with cell lists when using a cartesian rather than a periodic distance policy.
Some more tests on the input to the cell list iterator class are added.
The provided distance policy is used explicitly to determine which cells are in interaction range.
An implicit assumption in the function mapping particles to cells is removed.
Cpp tests are adapted for consistency.
In particular, for cartesian distance policy in a finite box, particle coords are shifted such that they are in the box.
since you're specifying xmin and xmax, is this assuming a cubic box?