Support nested entities

We need to support nested entities with a custom BIO format:

Charles B-child B-name
né I-child
à I-child 
Beaune I-child B-location
en I-child
1836 I-child B-date
père O
Jean B-father B-name
Bigre I-father B-surname
charpentier I-father B-occupation
de I-father
cette I-father B-location
paroisse I-father I-location
mère O
Marie B-mother B-name

We could define a hierarchy between entities:

levels:
  0:
    - child
    - father
    - mother
  1:
    - name
    - surname
    - location
    - date
    - occupation

We need to modify the Tokens / Spans / Document classes to support this new format, and also define a new property named hierarchy that would return a dictionary (input of David's metrics):

{
    "hierarchy": [
        {
            "category": "child",
            "children": [
                {
                    "category": "name",
                    "children": [
                        "Charles"
                    ]
                },
                {
                    "category": null,
                    "children": [
                        "n\u00e9"
                    ]
                },
                {
                    "category": null,
                    "children": [
                        "\u00e0"
                    ]
                },
                {
                    "category": "location",
                    "children": [
                        "Beaune"
                    ]
                },
                {
                    "category": null,
                    "children": [
                        "en"
                    ]
                },
                {
                    "category": "date",
                    "children": [
                        "1836"
                    ]
                }
            ]
        },
        {
            "category": "father",
            "children": [
                {
                    "category": "name",
                    "children": [
                        "Jean"
                    ]
                },
                {
                    "category": "surname",
                    "children": [
                        "Bigre"
                    ]
                },
                {
                    "category": "occupation",
                    "children": [
                        "charpentier"
                    ]
                },
                {
                    "category": null,
                    "children": [
                        "de"
                    ]
                },
                {
                    "category": "location",
                    "children": [
                        "cette"
                    ]
                },
                {
                    "category": "location",
                    "children": [
                        "paroisse"
                    ]
                }
            ]
        },
        {
            "category": "mother",
            "children": [
                {
                    "category": "name",
                    "children": [
                        "Marie"
                    ]
                }
            ]
        }
    ]
}