Providing a default value to DatePeriodField

[hsmett]

The DatePeriodField model field can't currently receive a default value (neither a DatePeriod object nor a json string)
Adding the functionality would be great, e.g. :

inspections_delta = DatePeriodField(
    verbose_name="Un contrôle technique est effectué tous les",
    null=True,
    blank=True,
    default=YearsPeriod(4),
)