ENH: Improve readability of MRML tests using vtkNew where possible

Simplify code removing tests for null object already being done
in the helper macro "EXERCISE_BASIC_OBJECT_METHODS"

For example, the following should not be needed:

vtkNew<vtkFoo> foo;
if (foo.GetPointer() == 0)
{
std::cerr << "Failed to instantiate vtkFoo" << std::endl;
}

Instead, the following should be used:

EXERCISE_BASIC_OBJECT_METHODS(foo.GetPointer())

Remove use of unnecessary vtkSmartPointer to check return value. Indeed,
since the operator "==" of the vtkSmartPointer is overloaded to compare with
the pointed object, there is no need to used a smart pointer. [1]

// ---------
vtkSmartPointer<vtkMRMLDisplayableNode> pnode = node1->GetDisplayableNode();
std::cout << "GetDisplayableNode returned "
<< (pnode == NULL ? "null" : "not null")
<< std::endl;
// ---------

has been changed into

// ---------
vtkMRMLDisplayableNode* pnode = node1->GetDisplayableNode();
std::cout << "GetDisplayableNode returned "
<< (pnode == NULL ? "null" : "not null")
<< std::endl;
// ---------

[1] https://github.com/Kitware/VTK/blob/0e0674a3695fdeaf01ce87dfc04e32111b7402e0/Common/vtkSmartPointerBase.h#L104

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22538 3bd1e089-480b-0410-8dfb-8563597acbee