Style: Reordering includes, move include to cpp.
Root of property tree is now a class attribute.
Partition write() into buildPropertyTree() and write().
Set default precision, that can be overriden.
Using the internal stream of Writer.
Correcting docu.
Add scalar properties to BoostVtuInterface.
[IO] Delete unused polys and surfaces.

If the allocated vectors are not added to the geo_objects object,
the memory is not released. Both the data and names are managed
by the GEOObjects class.
Added documentation to buildPropertyTree().
[BoostVtuInterface] Removed unused attribute.
Declaration of _mesh moved to declaration of other attributes.
Made all protected declarations private.
Fixed spelling.
BoostVtuInterface::setMesh(): Return if mesh is nullptr.
Closed polylines do not need the first vertex twice.

This change also introduces line fbx export to Unity
in the VtkFbxExporter submodule.
Simplified.
Merge pull request #377 from TomFischer/AddFunctionalityToBoostVtuInterface

Add functionality to boost vtu interface for properties
Merge pull request #380 from bilke/line-source-fix

[GUI] Fix for VtkPolylineSource