Add element volume computation to GeometryFM3D

[ecomodeller]

Context

Element volumes are needed for 3D layered meshes. The building blocks already exist in the codebase — this is mainly an assembly task.

What exists today

• GeometryFM2D.get_element_area() — horizontal areas, handles geographic coordinates (lon/lat → m² via cos(lat) correction)
• _GeometryFMLayered._calc_dz() — element heights from node z-coordinates
• elem2d_ids — maps each 3D element to its 2D footprint
• _zn on DataArray — time-varying node z-coordinates (n_timesteps, n_nodes)

Approach

Formula: volume = 2D_area * dz — exact for MIKE FM prismatic elements (vertical extrusion).

Static volumes (both projected and geographic)

areas_2d[elem2d_ids] * dz. Geographic coordinates only affect horizontal area, which get_element_area() already handles. Z is always in meters.

Dynamic (time-varying) volumes

2D area is constant over time, only dz changes per timestep. The _zn array lives on DataArray, not geometry, so:

• Static property element_volumes on GeometryFM3D using static z
• Method calc_element_volumes(zn=None) accepting optional dynamic zn (same pattern as _calc_dz)
• Consider convenience on DataArray that passes _zn through automatically

Complexity summary

Scenario	Complexity	Notes
Static + projected	Trivial	Multiply existing quantities
Static + geographic	Trivial	Already handled by get_element_area()
Dynamic + any CRS	Moderate	Need _zn data, API design question

Verification

MIKE 3 engine can produce a volume item — use as ground truth for both sigma and sigma-z meshes, including dynamic cases with wetting/drying.