Hierarchical data output

[changliao1025]

Since the model is using the hierarchical data model to calculate the attributes of each cell, it is desirable to update the output structure.
For example, the model can output individual segments/subbasins/watersheds.

[changliao1025]

Ideally, we can use the json library to output them directly in json format. Or we can use text-based and let the Python frontend to convert to json.

[changliao1025]

1. calculate segment characteristics
2. calculate segment travel length
3. calculate subbasin characteristics
4. calculate subbasin travel length
5. calculate subbasin cell travel length
6. update cell attributes

[changliao1025]

Stream segments may also be output individually. This will be used by the Python frontend for the segment operation.