Refactor integration test suite to test for memory leaks

Referring to #188, the integration test suite must be refactored so that each test is executed to test for memory leaks.

Dependencies to make this happen:
- pyobgraph
- VTK egg which as debug leaks trace facility enabled (CMAKE setting: -DVTK_DEBUG_LEAKS)

It should be noted that vtk debug leaks falsely reports leaks. If you have the first line in your program as:
`vtkDebugLeaks.PrintCurrentLeaks()`, it will report leaks similar to:

` vtkDebugLeaks has detected LEAKS!
- Class "vtkOutputWindow" has 1 instance still around.
- Class "vtkObjectFactoryCollection" has 1 instance still around.
- Class "vtkBoxMuellerRandomSequence" has 1 instance still around.
- Class "vtkMinimalStandardRandomSequence" has 1 instance still around.`

These are not really leaks, but these are reported by static initializers and are the cost of linking against VTK :-). So the test will have to account for these, the most elegant solution is to update 'vtkDebugLeaks`itself or a class that wraps around`vtkDebugLeaks` and handles this use case.