Example needs to add the river class

[kasra-keshavarz]

If available, MESH can understand the river classes defined for a setup. For example, in MERIT-Basins, the order value could be used as the IAK in the drainage database. This could be informative for new users using this workflow.

[kasra-keshavarz]

Addressed in iss21 branch, will make a PR soon and close this.