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

http://stackoverflow.com/questions/17779982/cost-of-passing-by-reference-when-a-template-type-is-fundamental

It can be that `const int var` can better optimized than `const int& var`