Defer setting plot backend as much as possible

This MR patches the gwinc.ifo command-line interface to defer setting the matplotlib backend until we know whether we will use show or savefig.

Out of curiosity, why use agg for saving? And what if the user wants to both save and display - is it ok to use agg then?

Yeah this works fine. This is what we're doing for the main cli script as well.

approved this merge request

merged

mentioned in commit 900fd7cc