Implement qswat_populate_gwflow_gis() for GIS-dependent gwflow tables

[Copilot]

The R package lacked GIS processing capability to populate the spatial gwflow database tables that the Python QGIS plugin generates via GWFlow.run(). This adds qswat_populate_gwflow_gis() using sf and terra to replicate that pipeline end-to-end.

New function: qswat_populate_gwflow_gis()

Accepts a project object plus paths to a conductivity shapefile, aquifer thickness raster, and optionally tile-drain / lake shapefiles. Populates:

Table	Source
gwflow_zone	Conductivity polygons clipped to basin
gwflow_grid	Fishnet cells with elevation, thickness, zone, boundary/active status
gwflow_rivcell	sf::st_intersection of streams → length per (cell, channel)
gwflow_lsucell	LSU polygon intersection → area per (cell, LSU); when hruorlsu_recharge ≥ 2
gwflow_hrucell	HRU polygon intersection → area per (cell, HRU); when hruorlsu_recharge is 1 or 3
gwflow_fpcell	Intentionally empty — no floodplain/upslope split in rQSWATPlus
gwflow_rescell	gis_water JOIN gwflow_lsucell + optional lake shapefile

Also updates gwflow_base.row_count / col_count.

Key implementation details

• Fishnet IDs assigned in raster order (left→right, top→bottom) via O(n) column-major-to-row-major conversion — no centroid extraction needed.
• GLHYMPS auto-detection: if logK_Ferr_ column is present, converts log10(k_intrinsic [m²]) × 100 to hydraulic conductivity in m/day; otherwise accepts any numeric column via conductivity_column.
• NoData fill uses two focal-mean passes (3×3 then 5×5) to approximate gdal.FillNodata(maxSearchDist=5).
• Multi-zone cells: zone assigned by mean of intersecting zone IDs, matching Python's groupby('Id').mean().

cfg     <- qswat_read_gwflow_config()
project <- qswat_setup_gwflow(project, gwflow_config = cfg)
project <- qswat_populate_gwflow_gis(
  project           = project,
  gwflow_config     = cfg,
  conductivity_file = "GLHYMPS_basin.shp",   # logK_Ferr_ auto-detected
  thickness_file    = "aquifer_thickness.tif" # cm → m via thickness_scale = 0.01
)

Tests

Eight new tests in test-gwflow.R use synthetic spatial data (1 km² square basin, flat DEM, uniform thickness raster, single GLHYMPS polygon, one stream) to verify grid dimensions, elevation/thickness values, zone K conversion, river/LSU/HRU cell counts, and recharge-mode gating (modes 1, 2, and 3).

[tadhg-moore]

@copilot can you integrate the new gwflow function with the workflow? So that they are executed if "use_gwflow" is set to True