Read operator: extension time-lagged ensemble handling

At the moment the read operator loads deterministic and ensemble data and assigns a realisation coordinate (member number). For deterministic this is 0.

For ensembles run with time-lagging this tickets adds capability to
- extract data for same validity times from time lagged runs to return a time-lagged cube including members that have run at different lead times.
- for time lagged cubes extract data for the same forecast length periods/lead times to enable performance assessment with lead times
- for time lagged cubes ability to define the forecast_reference_time depending on a chosen cycle time (last cycle)
- extract validity time, forecast_reference_time, forecast_period

@SGallagherMet: adding you here to ensure we include capability needed from your side