Matplotlib backend set to Agg by default

[ieivanov]

utils.plotting always calls matplotlib.use('Agg') which changes the matplotlib backend. This is a problem when importing plotting functions into Jupyter notebooks, which often use a different backend. A workaround is to change back the matplotlib backend in the Jupyter notebooks after importing reconstruct-order functions. It'll be better to set the matplotlib backend in reconstruct-order only when writing images through reconstruct-order.