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

here you state the fact that delta is possitive twice
`is always positive and <3/2` and `means |delta| == delta`
and `|delta| < 3/2` only means `-3/2 < delta < +3/2`