``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.