Add Hash for MeshGrids so that one can check mesh equvalence on the fly

[kunyuan]

• design a hash function that calculates a hash number for a given mesh type and mesh points.
  - Of course, it will be best for each mesh type to implement their specialized hash function
  - If not, a generic hash function can be used based on the iterator interface.

• Add meshash field in MeshArray struct

• Based on meshhash, now one can check meshes of the incoming MeshArrays in +,-,*,/ operations on the fly.