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

cloud shape is the shape (not assignment shape of the particle).  (@see Computer Simulation Using Paricles from Hockney and Eastwood)
Our current shape implementation is always the assignment shape (integral of the cloud shape)