Add `merge_cubes` openEO Process

[emmanuelmathot]

Objective

Implement the merge_cubes process in openEO by TiTiler for multi-sensor data fusion.

Specification

• Process page: https://openeo.org/documentation/1.0/processes.html#merge_cubes
• JSON spec: https://github.com/Open-EO/openeo-processes/blob/master/merge_cubes.json

Implementation References

1. Raster Stack Data Model: https://sentinel-hub.github.io/titiler-openeo/raster-stack/

   • Understanding how multi-cube merging works in TiTiler-openEO

2. Existing Implementation: aggregate_spatial in titiler/openeo/processes/implementations/spatial.py:L199

   • Follow similar patterns for parameter validation and callback handling

Implementation Steps

1. Copy specification

   • Download merge_cubes.json from openeo-processes repo
   • Place in data folder following existing process organization

2. Implement process

   • Merge two data cubes with optional overlap resolver
   • Handle dimension alignment (spatial, temporal, bands)
   • Support overlap resolver callback for conflicting values
   • Handle different spatial/temporal resolutions appropriately

3. Register process

   • Add to process registry following existing patterns

4. Testing

   • Unit tests for cubes with/without overlap
   • Test with different overlap resolvers
   • Test multi-sensor scenarios (Sentinel-1 + Sentinel-2)
   • Test temporal and spatial misalignment cases

Acceptance Criteria

• JSON spec copied to data folder
• Process implements specification correctly
• Handles dimension alignment per spec
• Overlap resolver works correctly
• Includes tests
• Multi-sensor fusion example