Combined distance policy

[gasteigerjo]

Using the periodic_distance policy everywhere introduces significant overhead, since this method is very central to the package. In the inner cells, a normal cartesian distance would suffice.
Furthermore, I am working on an additional boundary condition right now, that further complicates the distance calculation and introduces more overhead.

Therefore, I propose a combined distance policy. Using this policy, one (simple) distance policy for inner cells can be set and additional ones for each boundary (i.e. x, y and z, maybe even x+/x-, y+/y- and z+/z-). Using a combined distance policy as an interface between CellLists and DistanceInterface reduces the refactoring necessary to a minimum and enables extensibility in the future.