Add in extra plot functionality needed for OSPAR assessments: auxiliary data plot, ratio plots, related time series plots.

[hansmy]

Initial release (January): already include functionality for reporting using function report_assessment. this currently produce meta-data, trend graph and data graph (also available with plot_assessment) and lay interpretation.

Issue for the February release: Add in extra functionality required for OSPAR assessment. This includes plot of auxiliary data, ratio plots, and plots of related time series.

[RobFryer]

Some of this functionality is also required to complete #140