Some module parameters do not have a value defined

[matt-graham]

As noted in #1419, there appear to be a few cases of parameters defined in Module.PARAMETERS (class) attribute that do not appear to have values defined in Module.parameters (instance) attribute after read_parameters method is called (for example init_or_higher_bmi_per_higher_wealth in Demography). We should fix these instance (either by removing parameter definition or specifying a value if appropriate and used elsewhere) as well adding automated tests to catch such instances (see #109).