lisa.__init__: Only set the matplotlib backend in headless setups
lisa.__init__: Only set the matplotlib backend in headless setups

Matplotlib spits some warnings when the backend in use expects a connection to
X11 server. Since the default needs it, set to Agg which does not operate with
windows.

credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa