Add CLI option to enable / disable displaying of auto-generated figures

Currently, when using --plot (or one of the other log-plot variants), the resulting figure(s) are always shown. This may not always be desirable, so an option named something like --no-show-plot should be added which will allow the automatic display of figures to be switched off. This flag should be False by default, but could be switched on when --saveplot is used.

This would also allow automated testing of plotting via the CLI, which is not possible / tricky at the moment as the figures will always be opened.