Use boost::variant for writable arguments in the IFC serializer

git-svn-id: https://svn.code.sf.net/p/ifcopenshell/svn/trunk@183 68180887-8925-40e4-aed3-789c086970ef
Tweak processing of IfcBooleanClippingResult and IfcPolygonalBoundedHalfSpace

git-svn-id: https://svn.code.sf.net/p/ifcopenshell/svn/trunk@184 68180887-8925-40e4-aed3-789c086970ef
Add revit2014_multiple_bounded_halfspaces.ifc test file

git-svn-id: https://svn.code.sf.net/p/ifcopenshell/svn/trunk@185 68180887-8925-40e4-aed3-789c086970ef
Cosmetic changes

git-svn-id: https://svn.code.sf.net/p/ifcopenshell/svn/trunk@186 68180887-8925-40e4-aed3-789c086970ef
Merge commit 'f58a38391e0333e0a2f2aafb791d5fbd8c89f715' (i.e. commit r186 from SVN)