Chargeconserving interpolator corrections and optimizations.

* Switched to normalized coordinates like in CIC.
* Corrected the interpolation s.t. it really fulfils the continuity equation now.
* Disabled Unit Test because it was testing for the wrong thing. (this algorithm does NOT calculate (x(t)-x(t-dt)) / dt!)  Check algorithm description for details.
* Removed boundary handling because this is done by the grid now. Periodic boundaries were hardcoded in the algorithm. This change might lead to a slightly higher probability of index out of bound errors.
* Removed the chargedensity variable from Particle because it is no longer needed.
* Renamed to ChargeConservingCIC to emphasize connection with CIC.