Add track origin selection for input variable plotting

This MR provides the functionality to require a specific track origin when plotting input variables. The default behaviour of plot_input_variables.py does not change, except for appending a suffix _All to the plot name (so the reference image names have also been updated). If desired, the user can now specify a list of track_origins in the configuration file. This will produce a set of plots, one for each track origin specified. Note that the truthOriginLabel variable must be present in the .h5 files used for plotting for this option to work.

The unit tests are currently failing because the x-axis labels are different to the reference plots (they now include the track origin in the x-axis title).