as on the C++ side the default for writing vtk files is 'ascii' - change to 'appendedraw'

I would like to change the default data type for the vtk writer when used through the python interface from 'ascii' to some binary data type to reduce the file size produced. It is still possible to change it to 'ascii' through a function parameter.

If nobody objects I would suggest merging this before the release to not have to change it after the first release of the python bindings in core.