Rewrite `subset` function to `clip` and only take the polygon

[rogerkuou]

Currently the subset function is designed as the MATLAB DePSI, which takes too many functionalities in one functions.

However, the threshold subset is more easily handled by where function in Xarray, and density dubset is already taken care by network_stm_selection in DePSI, therefore only the polygon croping function is needed for STM.

Therefore we can simplify this function only subseting by polygon. Since the function is changed, we can rename it to clip