Improvements for read_optional_key implementation

[SeanBryan51]

The read_optional_key function was added in PR #238:

benchcab/src/benchcab/config.py

Lines 74 to 121 in 5c526e4

	def read_optional_key(config: dict):
	"""Fills all optional keys in config if not already defined.
	The default values for most optional keys are loaded from `internal.py`
	Note: We need to ensure that the `name` key for realisations exists for
	other modules, but it doesn't have a default value. So we set it to
	`None` by default.
	Parameters
	----------
	config : dict
	The configuration file with with/without optional keys
	"""
	if "project" not in config:
	config["project"] = os.environ.get("PROJECT", None)
	if "realisations" in config:
	for r in config["realisations"]:
	r["name"] = r.get("name")
	config["science_configurations"] = config.get(
	"science_configurations", internal.DEFAULT_SCIENCE_CONFIGURATIONS
	)
	# Default values for spatial
	config["spatial"] = config.get("spatial", {})
	config["spatial"]["met_forcings"] = config["spatial"].get(
	"met_forcings", internal.SPATIAL_DEFAULT_MET_FORCINGS
	)
	config["spatial"]["payu"] = config["spatial"].get("payu", {})
	config["spatial"]["payu"]["config"] = config["spatial"]["payu"].get("config", {})
	config["spatial"]["payu"]["args"] = config["spatial"]["payu"].get("args")
	# Default values for fluxsite
	config["fluxsite"] = config.get("fluxsite", {})
	config["fluxsite"]["multiprocess"] = config["fluxsite"].get(
	"multiprocess", internal.FLUXSITE_DEFAULT_MULTIPROCESS
	)
	config["fluxsite"]["experiment"] = config["fluxsite"].get(
	"experiment", internal.FLUXSITE_DEFAULT_EXPERIMENT
	)
	config["fluxsite"]["pbs"] = internal.FLUXSITE_DEFAULT_PBS | config["fluxsite"].get(
	"pbs", {}
	)

Instead of hard coding default values for each field within the function, we can use the schema itself to set default values for missing fields in the document. The cerberus python package provides normalization rules which support this functionality. This approach would also lets us change default values for specific fields in the future without breaking the test suite.