Add plotting extension

Plotting extension : create png files containing plots for chosen monitoring channels.
We are not allowed to have a hard dependency on matplotlib anywhere. At the very least, this needs to be wrapped in a try/except block that disables the feature if matplotlib is not present.
I also think train.py is not the right place for this. Can't you put it in train_extensions/plots.py? scripts/train.py is meant to be maximally generic.