SpaceVaryingInput lake depth

[AlexisRenchon]

TO DO:

• make a plot of lake depth to make sure it looks good... if a pixel has IMERG mask (is a lake) but no depth, make it 10 m

• if era5 lake depth exist, consider that pixel a lake, even if IMERG doesn't

• use f (not binary) (IMERG should also be possible to convert to f?)

• Add ERA5 lake depth as a SpaceVaryingInput, replacing the previous constant 10 m depth. The depth field is read from the era5_lake_cover_and_depth artifact (dl variable) and stored on the SlabLakeModel struct.

• Add a tunable slab_lake_depth_scale_factor parameter (α, default 1) so that depth_slab = α × depth_ERA5. Where the IMERG mask identifies inland water but ERA5 depth is zero or missing, the depth falls back to slab_lake_depth (10 m).

• Parameterization functions (lake_energy_at_freezing, lake_liquid_fraction, lake_energy_from_temperature, lake_temperature) now take depth as a separate positional argument to support spatially varying depth.

• Add era5_lake_cover_path() and era5_lake_depth_path() artifact accessors.

• The inland water mask still uses the IMERG land-sea mask (ERA5 lake cover misses some water bodies like the Black Sea).