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/IfcGeomIteratorSettings.h

@@ -80,8 +80,10 @@ namespace IfcGeom
             APPLY_LAYERSETS = 1 << 13,
 			/// Search for a parent of type IfcBuildingStorey for each representation
 			SEARCH_FLOOR = 1 << 14,
+			///
+			SITE_LOCAL_PLACEMENT = 1 << 15,
             /// Number of different setting flags.
-            NUM_SETTINGS = 14
+            NUM_SETTINGS = 15
         };
         /// Used to store logical OR combination of setting flags.
         typedef unsigned SettingField;