Tracks to vtk on object

[shimwell]

Currently there is a script that one can run from the terminal on a tracks.h5 file.

This PR adds the ability to write vtk files of tracks from within the OpenMC Python API. This allows users to create the vtk file in the following manner

import openmc
tracks = openmc.Tracks('tracks.h5')
tracks.write_tracks_to_vtk('tracks.vtk')

Part of the motivation for this is to make the production of track vtk files a little easier from jupyter notebooks with out the need for ! to run scripts. ps I am also viewing the vtk files in jupyter so it can make a nice workshop example