fix: price interpolation (#1154)

Use forward fill to interpolate time series data that represents prices:
- elecprice_marketprice_wh
- feed_in_tariff_wh

Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>

tests/test_elecpriceimport.py

@@ -112,6 +112,7 @@ class TestElecPriceImport:
             start_datetime=provider.ems_start_datetime,
             end_datetime=provider.ems_start_datetime + to_duration(f"{len(expected_values)} hours"),
             interval=to_duration("1 hour"),
+            fill_method="ffill",
         )
         # Allow for some difference due to value calculation on DST change
         npt.assert_allclose(result_values, expected_values, rtol=0.001)