Read the ENTSO-E interim CSV(s) written by the downloader.
This is the library-level counterpart of the inline load_interim() helpers that every submission script defines identically. The two modes reproduce the exact CSV-reading logic from those scripts so callers can replace their local copies with a single import.
Which interim layout to read. "combined" (default) — reads <data_home>/interim/energy_load.csv (written by download_new_data). This is the single DE-total series path used by chronos, optuna, and spotoptim submission scripts. "four_zone" — assembles the per-zone QC frame via build_zone_qc_frame and also writes the zones-only modelling CSV (interim/energy_load_zones.csv). This is the normal four-zone path used by team4_4zones_submit.
'combined'
data_home
Root data directory. If None, resolved via get_data_home().