Object Memory: Allow specifying scene snapshot via standard ArmarX Data path (in addition to inside PriorKnowledgeData), similar to simulator

Closes #121 (closed)

This allows using the same strings in the Simulator and ObjectMemory (e.g. PriorKnowledgeData/scenes/R003.json), and allows loading scenes outside the single PKD package (e.g. project packages)

It should still work with bare scene names as before