Refresh the UW->badlands coupling

[julesghub]

Motivation: The current coupling is poorly documented, with few running examples and time stepping faults / bugs that have never been properly addressed (i.e. the fact that the 2 codes can get out of sync after several iterations).

Idea: @julesghub and @NengLu - refresh the coupling of UW and Badlands to remove the time stepping issues and build some better documentation.

1. Fix time stepping to conform properly at the end of an UW iteration.
2. Have more robust documentation about the coupling: highlight special variables that control coupling and detail the algorithm.
3. Create some examples show the coupling in action for different models.

We welcome other thoughts and ideas from the community.

[NengLu]

The initial model testing cases can be found at https://github.com/underworld-community/uwg-coupling-test.

Two 2D models are currently considered for testing:

• 2D Topography Relaxation Model
• 2D Wedge Model (modified from Thieulot et al., 2014)

We welcome suggestions for additional models to include in the testing suite.

[bknight1]

Is there a resource/guide for setting up the badlands xml file?

I wasn't able to work out what kind of range each parameter should be in when attempting the coupling with my orogenic models, as the one provided (which was a while ago) resulted in very fast erosion rates in my models.

[julesghub]

Is there a resource/guide for setting up the badlands xml file?

https://badlands.readthedocs.io/en/latest/xml.html

[julesghub]

Talk with @YangHaibin1102 .

• Predefined Underworld Topography model idea.
• Badlands input file needs information on boundary conditions (https://badlands.readthedocs.io/en/latest/xml.html#grid-structure)
• Alignment of tectonics timestep and surface processes timestep.