Site local placement (#240)

* ifcgeom: Add placement_rel_to for convert()

Change geometry conversion in `convert()` for `ObjectPlacement` such
that we can stop the `PlacementRelTo()` recursion when we hit an
`ObjectPlacement` that places a certain type, e.g. a `IfcSite'.

Introduce a new IteratorSetting `SITE_LOCAL_PLACEMENT` which will be
exposed in `IfcConvert`.

* IfcConvert: Add --site-local-placement

Some IFC authoring tools may put the global origo at a large distance
away from the actual building geometry. Also, the global X- and Y-axes
may be at arbitrary angles with natural building axes. Using
--center-model or --model-offset may place the converted geometry closer
to origo, but will still use the global IFC definition of axis
directions. Also, in a scenario with multiple IFC files for one
construction site, using --center-model on each of them would not
generate consistent geometry across OBJ or DAE files.

Solve this by introducing --site-local-placement which will disregard
the ObjectPlacement at the IfcSite level.

src/ifcgeom/IfcGeom.h

@@ -99,6 +99,10 @@ private:
 	std::map<int, SurfaceStyle> style_cache;
 
 	const SurfaceStyle* internalize_surface_style(const std::pair<IfcSchema::IfcSurfaceStyle*, IfcSchema::IfcSurfaceStyleShading*>& shading_style);
+
+	 // For stopping PlacementRelTo recursion in convert(const IfcSchema::IfcObjectPlacement* l, gp_Trsf& trsf)
+	IfcSchema::Type::Enum placement_rel_to;
+
 public:
 	Kernel()
 		: deflection_tolerance(0.001)
@@ -109,6 +113,7 @@ public:
 		, ifc_planeangle_unit(-1.0)
 		, modelling_precision(0.00001)
 		, dimensionality(1.)
+		, placement_rel_to(IfcSchema::Type::UNDEFINED)
 	{}
 
 	Kernel(const Kernel& other) {
@@ -313,6 +318,8 @@ public:
 #endif
 	}
 
+	void set_conversion_placement_rel_to(IfcSchema::Type::Enum type);
+
 #include "IfcRegisterGeomHeader.h"
 
 };