[BUG] Heat_pump_imort is pointing to wrong schema for etrago-data

[MoritzSchloesser]

Describe the bug
When importing heat pump data from a local database, the tables egon_etrago_bus and egon_etrago_link are looked for in the schema 'supply'. The tables are stored in the schema 'grid'. Changing the schema in the file 'heat_pump_import.py' should resolve the error.

The same error should occur whene loading data from the OEP since the tables are stored in the schema 'grid' there as well.

[joda9]

is fixed with #613