Update 2021.9.7

[cindytsai]

Update 2021.9.7

• Fix bug MPI_Gatherv not support send count > INT_MAX.
• Simplify choosing data dimension and data type between data_dim and grid_dimensions, data_dtype and field_dtype. This is now all done inside append_grid.cpp.
• If data_ptr = NULL, libyt shouldn't abort when data_dtype or field_dtype is not set and check_data==false. We don't need to wrap this array, hence no need to set data type.
• Support more particle type, YT_LONG.
  • User can also extend their new data type, see Add New Particle Data Type.
• Update libyt/example to match newest yt.
• New function get_npy_dtype to match yt_dtype to NumPy Enumerate Type. So that user can add new data type more eifficiently.
  • See how to add new type at Add New NumPy Enumerate Type.
  • Use Enum Loop to check if the input yt_dtype is valid or not.