Add plotting extension

Plotting extension : create png files containing plots for chosen monitoring channels.
I really don't think you should duplicate summation logic all over the place. Just have the plotting functionality do plotting. If you need the sum of two monitoring channels, implement that somewhere else, and plot the result.

The history of the library has always been that somebody implement summing in some peripheral part of the library, then we later realized that summing was complicated and needed to be centralized. For example, sgd.py used to take a tuple of Costs and it would sum them, but then we got rid of that and made the SumOfCosts class to handle all the logic of the spaces, get_gradients calls, etc.