Submodels for FeliX

[RyanTanYiWei]

Could be nice to have a sub-model structure based on the stable global version of FeliX to enable parallel developmental workflows.

[yequanliang1993]

@RyanTanYiWei , hi Ryan, I just created a branch for this issue. You can start to work on the issue in that branch. The only thing right now is you cannot push codes into that branch I guess. But please commit any changes in your local machine.
Let me know if you have any problem.