Allow complex node paths in `dbschema.json`

```
In `maps`, `*/data.json`, `to_table`:
{
"node": "topics", # Allow more complex paths in form of array
# "node": [], --> The whole file object has to be used in the table
# "node": ["main", "topics"] --> The node data["main"]["topics"] has to be used
"table": "topic"
}
```

Simply allow `to_table:node` to be either a string or an array of strings.
In case of the array the multiple strings specify the path from the root to reach the target to include in the table.

This would allow to use frameworks that natively store namespaced data (i.e. path of two nodes) as for example does [Mavo](http://mavo.io).

Start by locating the code that reads dbschema.json and processes maps entries with to_table and node. Trace how a string node selects data, then check the existing schema-related tests or fixtures if available. Done means supporting an empty array, a single-element array, and nested string paths without breaking the existing string form.

Scritto dal modello di indicizzazione a partire dal testo della issue.