Add Coweeta watershed example and workflow utilities

[rfiorella]

This PR adds a new demonstration case for the Coweeta LTER watershed site and supporting infrastructure for ELM-ATS coupled simulations.

New example:

• examples/coweeta/: Complete watershed simulation setup
  • Mesh (.exo), domain metadata (.nc), surface data (.nc), subsurface properties (.csv)
  • Full ATS XML configuration and ELM namelist (user_nl_elm)
  • Build script following project conventions

Workflow integration:

• Adds Watershed Workflow as a submodule (watershed_workflow/) for mesh/metadata generation

Data conversion utilities:

• scripts/atsify_surfdata.py: Converts ELM surface data files to ATS+ELM compatible format
• scripts/elm_to_xdmf.py: Converts ELM history files to XDMF for VisIt/ParaView visualization
• Updates to scripts/validate_surfacefile.py for new format

Note: The conversion scripts (atsify_surfdata.py, elm_to_xdmf.py) were AI-generated prototypes and should be validated before production use.

Improvements to existing examples:

• Standardized build_example.sh across Oak Harbor cases
• Updated XML configurations for Oak Harbor transect examples
• Removed unused elm_output_data references from namelists

Submodule updates:

• E3SM, Amanzi, inputdata submodules updated to support Coweeta case
• Watershed Workflow submodule added and configured