The new visualizer allow to change the visualization, e.g. only visualize one component or visualize the sum of all components

This makes creating temporary ConstantGridFunctions as default arguments
easier, using ConstantFunction + as_grid_function causes problems as the
resulting GridFunction only stores a reference to the ConstantFunction

Fix install of python package

closes #56

[spe10] use data files from dune-xt-data

new-master