[CF-metadata] Example of forecast data

[Jonathan Gregory]

In fact, it is indeed what we thought of before at the meeting in PCMDI 1.5
years ago :-)

(iv) Multiple validity and analysis times, where not all cases exist, for
instance forecasts out to a given validity time from successive analyses. This
can be done by using a single index dimension, with validity and analysis time
coordinates having this dimension pointed to by the coordinates attribute.

Jonathan