Stream segment output

[changliao1025]

The backend will now output a json file which contains all the stream segments for each watershed.

[changliao1025]

The post process will convert such a json file back to geojson in edge mode. The pyflowline model will be reused to generate the segment based geojson.

[changliao1025]

The edge json may contain segment index information, which could be used for geojson.