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.
Merge pull request #377 from TomFischer/AddFunctionalityToBoostVtuInterface

Add functionality to boost vtu interface for properties