Plot points with `.point_data`

[stefsmeets]

From #259:

Add function to plot points with data from MeshContainer.points_data. Probably needs a new data class to get those data: Points

mesh = MeshContainer('phi.vtk')
mesh.get('points')  # --> `Points(...)`
mesh.plot('points', key='phi')  # --> Scatter plot colored by points

Can be demonstrated in the horse example for sfepy.