get_prop has incorrect default components

[dallan-keylogic]

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As noted in #1613 , get_prop for the natural_gas_pr.py properties in models_extra incorrectly writes:

    if components is None:
        components = list(_component_params.keys())

for the default components. The correct statement is

    if components is None:
        components = list( comp for comp in _component_params.keys())

This error should be remedied and a test should be added.