Forecast Reference Time

Requirement:
* Output files from LFRic define the `forecast_reference_time` for the forecast
* https://vocab.nerc.ac.uk/standard_name/forecast_reference_time/

This is complementary to the `time` coordinate, which is output from the model (noting #423)

For a single forecast, the definition of a scalar value of `forecast_reference_time` and a coordinate of `time` enables the `forecast_period` to be unambiguously derived from output data.

Out of scope:
* It is not intended to provide an explicit `forecast_period` coordinate. This is in part due to its derivability and in part due to technical limitations in XIOS that make this challenging to output from the simulation.