fix(netcdf): Fix context time attributes

[gmertes]

Description

Two bugs:

1. The netcdf output uses the context attributes self.lead_time and self.time_step in the open routine, which is called before these attributes are set.
2. NetCDFOutput.open reads reference_date from the context, but doesn't check if the value is not None.

• Set the context attributes self.lead_time and self.time_step at the beginning of execute, before they are accessed.
• Check if the context reference_date is not None, otherwise use the date from the state.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update