 ``duration`` converter, to display integral or fractional values as
    human-readable time spans (e.g. 1.5 as "1 hour 30 minutes").

    Can be used on any numerical field.

    Has an option ``unit`` which can be one of ``second``, ``minute``,
    ``hour``, ``day``, ``week`` or ``year``, used to interpret the numerical
    field value before converting it. By default use ``second``.

    Has an option ``round``. By default use ``second``.

    Has an option ``digital`` to display 01:00 instead of 1 hour

    Sub-second values will be ignored.
    