Make time_coverage_start/end attributes optional

[sfinkens]

The global time_coverage_start/end attributes are looked up in metadata.TIME_COVERAGE. If a platform is missing in that dictionary, pygac-fdr-run fails with a KeyError. This should not be a show stopper. Instead these attributes should be omitted in that case. Proposed solution: Replace

pygac-fdr/pygac_fdr/writer.py

Lines 351 to 353 in a270843

	time_cov_start, time_cov_end = TIME_COVERAGE[
	get_gcmd_platform_name(ch4.attrs["platform_name"], with_category=False)
	]

with TIME_COVERAGE.get(..., (None, None)), and add the attributes only if not None. Else a warning would be useful.