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

`must` -> `have to`
`a nvcc` -> `an nvcc` (because you start with a vowel when you pronounce `nvcc`)
`We have to use float as loop variable due to an nvcc bug`