match subtrees by name in ImportTreeFromJSON

[aysha-afrah26]

Export/import round trip on current master, from the regression test added here:

tests/gtest_blackboard.cpp:782: Failure
Expected equality of these values:
  blackboard->get<std::string>("owner")
    Which is: "alpha"
  expected_owner[i]
    Which is: "main"

ImportTreeFromJSON writes the n-th JSON object into tree.subtrees[n], but ExportTreeToJSON keys the document by subtree name and nlohmann::json iterates keys alphabetically, so the blackboards are restored into the wrong subtrees unless the names happen to sort in creation order (Sub::10 already sorts before Sub::9). Each subtree is now looked up by the name the exporter writes; a document missing one of them throws std::runtime_error where it used to be applied by position.