Support on grids and efficiency

[henry2004y]

Currently most tracing functions in the package assume regular grid, meaning that the size of each cell is not changing. However, the whole grid information is passed all the way down to the kernel functions, which may be a waste of memory.

In the future we may want to extend the functions to support more grid types, so be careful about changing the kernel functions. We need to find a balance between generalization and efficiency.