Pyvista

This release includes additional structured mesh methods, particularly the ability to check the cell quality directly from the grid object as well as speeding up the in-place rotation of the meshes and grids.

Examples have also been updated to reflect the changes in the API.

The implementation prior to this release was to instantiate a ``vtk`` object and then add methods to the existing objects to extend the functionality of the vtk objects. It worked, but it was messy and lead to the objects not being garbage collected as these methods were viewed by Python as references. There are now three unique subclasses for ``vtkInterface``:

- ``vtkInterface.PolyData``
- ``vtkInterface.UnstructuredGrid``
- ``vtkInterface.StructuredGrid``

Additionally, there are now the private classes ``Common`` and ``Grid`` that include methods in common with two or all three of the aforementioned subclasses. This eliminates code duplication while preserving functionality.

Each of these objects has help and documentation outlined in [readthedocs](vtkinterface.readthedocs.io/en/latest/).

Code is stable enough at this point to call this a release.

See documentation at http://vtkinterface.readthedocs.io/en/latest/ for notes and details on using ``vtkInterface``.