add new current solver zigzag

add new method to calculate the current of moving particles

- [x] arbitrary shape support
- [x] 2D support
- [x] 3D support
- [x] documentation

Tested:
KHI electrons/ positrons
- [x] 2D CIC/TSC/PCS
- [x] 3D CIC/TSC/PCS

Where does "cellSize" come from? Is it a private member (=> this->)? It would be nice to recognize its origin easier.