Add totalized demand curves to eTraGo tables

[IlkaCu]

The demand curves for

• households
• cts
• industry

are stored in interim tables and need to be summarized per bus and added to the eTraGo tables.

[IlkaCu]

In the current dev branch electrical demand curves are stored in the following tables:

• demand.egon_osm_ind_load_curves --> industrial demand curves per osm landuse area
• demand.egon_sites_ind_load_curves --> industrial demand curves per industrial site
• demand.egon_etrago_electricity_cts --> CTS demand curves per substation

Household demand curves are still missing (see PR #260) and need to be added to the methodology as soon as the data is available.

Relevant columns for the merge are subst_id, scenario and p_set