Make PV system dropout type hint optional (#297)

Make PV system dropout optional in config

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Sukh-P

ocf_datapipes/config/model.py

@@ -156,7 +156,7 @@ def dropout_fraction_valid(cls, v):
 class SystemDropoutMixin(Base):
     """Mixin class, to add independent system dropout"""
 
-    system_dropout_timedeltas_minutes: List[int] = Field(
+    system_dropout_timedeltas_minutes: Optional[List[int]] = Field(
         None,
         description="List of possible minutes before t0 where data availability may start. Must be "
         "negative or zero. Each system in a sample is delayed independently from the other by "

ocf_datapipes/load/nwp/providers/excarta.py

@@ -1,8 +1,8 @@
 """Excarta Loading"""
+
 import numpy as np
 import pandas as pd
 import xarray as xr
-import datetime
 
 from ocf_datapipes.load.nwp.providers.utils import open_zarr_paths