Add option to delineate subbasins using either streamorder of Pfafstetter thresholds

[markhegnauer]

In regions with no / or limited number of gauges, it's sometimes still useful to derive subbasins for analysis and recognizability by users / clients. In Pyflwdir there are already options to do this (https://deltares.github.io/pyflwdir/latest/basins.html), but these functions are not yet available through HydroMT.

I would suggest implementing this in a way that the user, through the inifile can set this up:

[setup_subbasin]
subbasin_method = gauges / pfafstetter / streamorder / minarea / ... (so either user specified location, or automatic generation)
subbasin_method_threshold = 5 (for the automatic methods)

Linked to: Deltares/hydromt_wflow#118

[hboisgon]

Nice suggestion thanks @markhegnauer !
I suggest to add these methods to flw.py and then access them in the setup method in hydromt_wflow (similar to basin_map that is already available)

[DirkEilander]

This also links to #159