Support multiregion function-object outputs in data reader

[blttkgl]

simple_function_object_reader() currently assumes function-object outputs live directly under postProcessing/<functionObject>/<time>/.... That breaks for multiregion OpenFOAM cases, where outputs are often nested as postProcessing/<region>/<functionObject>/<time>/....

This causes lookups like fluid/cylinderT to fail even when the data exists on disk.

Expected behavior:

• Discover multiregion outputs during function-object scanning
• Allow reads using names like fluid/cylinderT
• Accept either fluid/cylinderT or /fluid/cylinderT
• Keep existing single-region behavior unchanged

Validation:

• Add unit coverage for multiregion discovery
• Add unit coverage for simple_function_object_reader("/fluid/cylinderT")

coolingCylinder2D.py