Custom extent class type has incorrect interval field type

[ernstdevreede]

The interval field in the Custom class (in extent.py) has the type interval: List[str].
According to the standard this should be: interval: List[List[str]] or even better interval: List[List[str|float]]

See: https://docs.ogc.org/is/19-086r6/19-086r6.html#_a2113fbe-7d9d-4295-bdfa-25b9cc89fa03 where the schema states:

type: object
required:
    - id
    - interval
    - reference
properties:
    id:
        type: string
    interval:
        type: array
        minItems: 1
        items:
            type: array
            minItems: 2
            items:
                anyOf:
                    - type: string
                    - type: number
            nullable: true
...