BUG: Fix vtkMRMLModelNode ApplyTransform & GetMesh

Reverts regression introduced in r25660 [1] by applying the transformation
to the mesh data object directly if the mesh was set with SetAndObserveMesh
(which uses vtkTrivialProducer to generate its mesh connection) and was not
set as a mesh connection being part of a pipeline.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=25660

Also call vtkAlgorithm::Update() in vtkMRMLModelNode::GetMesh() to
ensure the mesh connection is up to date when getting its output data
object.

Also remove changes to MeshType and calls to Modified() in GetPolyData()
and GetUnstructuredGrid(). Those were triggered when data inconsistency
was detected, but a Get() method should never modify an object. Keep
warnings only.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25681 3bd1e089-480b-0410-8dfb-8563597acbee