Add derivation from em_dense_reconstruction_dataset to cell_morphology

[GianlucaFicarelli]

About naming:
em_dense_reconstruction_dataset_cell_morphology is just the concatenation of the used and generated entity types as for emodel_circuit.
Alternatives:

• use a name that describes the purpose of the derivation as em_skeletonization
• use unspecified as it is used for electrical_cell_recording to emodel, but it would be harder to filter by type

For reference, the existing derivation types are:

• circuit_customization (circuit to circuit)
• circuit_extraction (circuit to circuit)
• circuit_rewiring (circuit to circuit)
• emodel_circuit (emodel to circuit)
• unspecified (electrical_cell_recording to emodel)

Fix #604
cc @MWolfR

[codecov]

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