Implement STEP export

[simonmoreau]

This pull request implements the export of a Model to an IFC (STEP) file. It adds the method Model.ToSTEP(ref Dictionary<Guid, int> indexDictionnary), whitch need an indexDictionnary to keep track of the relation between STEP index and Guid. It also implements a series of STEPValue extentions (in the namespace STEPExtensions) on various types to describe how these types should be exported in STEP. It also implement the corresponding test ExampleModel_Serialize_STEP(). This pull request follow another one in IFC-gen.