interpretation of IDS strings to value types (xs:base types)

[CBenghi]

We need to document the suitability of xs:base types to the IFC data types.

@pasi-paasiala posted on slack a list of valid xsd types, that can be used in the restrictions.

My view is that we need to map explicitly the interpretation of IDS strings to value types depending on IFC type.

Analysing the values of attributes and properties in the various schemas (for the purpose of IDS audit tool) I think the list is fairly short, but we need to add a set of test cases to ensure they are all covered.

[CBenghi]

Closed with #291