feat: Write node metadata to json

[antoneri]

The metadata key is an object to enable multiple metadata dimensions, and add named metadata categories in post-processing.

The metadata value is a string to highlight that this is a categorical variable. In Alluvial, we also support numerical metadata (from e.g. color map equation) and we distinguish between categorical and numerical by the value types.

Metadata is only being written for memoryless networks as we don't support using both memory and meta map equation at the same time.

Output:

"nodes": [{
      "path": [1, 1],
      "modules": [1],
      "name": "2",
      "flow": 0.214286,
      "mec": 0.230673,
      "metadata": {"0": "1"},
      "id": 2
    },
]