Expand/fix usage of async for SUMO access

[sigurdp]

PR #378 incorporates usage of async in many/most of the access layer functions, but we should revisit this to ensure that we use async calls all the way where possible.

Some exampler of functions that should be refactored are:

• SurfaceAccess.get_realization_surface_data()
• SurfaceAccess.get_statistical_surface_data()
• SurfacePolygonsAccess.get_surface_polygons()

Some of these are non-async now, while others are defined as async, but do not actually use underlying async calls to Sumo.

Another related topic is whether we should have dual (both sync and async) functions for all or some of these access functions.
While seeking performance optimizations, we have done several experiments involving Python multiprocessing, and in that case, having async functions quickly becomes a nuisance, since by default there is no event loop running.

In that case, how should we name the functions?
We may for example need both:
SurfaceAccess.get_realization_surface_data_sync() and SurfaceAccess.get_realization_surface_data_async()

as well as both:
SurfaceAccess.from_case_uuid_sync() and SurfaceAccess.from_case_uuid_async()

etc.