Add MOGREPS-G forecasts reader for GHI

[yyoon4]

Add MOGREPS-G (Met Office Global and Regional Ensemble Prediction System - global ensemble), version PS45, medium-range forecasts reader for GHI

Brief information:

1. Spatial resolution: approximately 20 km
2. 18 ensemble members
3. Forecast length: up to 8 days

Reference website:
https://www.metoffice.gov.uk/research/weather/ensemble-forecasting/mogreps

Note that MOGREPS-G is used as proxy forcing instead of GALWEM-GE

Recent MOGREPS-G datasets are archived here:
/discover/nobackup/jwegiel/LISFV7.5_Deliverables/MR/MOGREPS-G