Describe n-ary relations

[webhart]

n-ary relations are cruical in MSE properties, e.g. the property "thermal expansion coefficient" depends on the temperature. The PMDco documentation should be extended to include a best-practice pattern for a typical n-ary relation and use this pattern in the "Boiling an egg" example, e.g. for describing the water boiling temperature denpendent on the altitude of the cooking location (sea-level or mountain top).