Exposure correction for MapDataset.to_region_map_dataset()

[Astro-Kirsty]

Description
When utilising MapDataset.to_region_map_dataset(region) if the region passed is not fully contained inside the geometry, the exposure needs to be corrected (take into account the fraction of the region contained inside the geom).
The IRF result will otherwise be incorrect, either over or under-estimated

Expected behavior
It is unclear how we would like to proceed, therefore discussion is needed. We can add to the agenda of the next dev call