Add method to overlay only some ratios in LineRatioPlot

[mpound]

Based on email from Carl F.

When using "LineRatioPlot" and "overlay_all_ratio" to make plots like the following

Is there a way to only select some of the ratios to plot? I ask because in this case the Cii/FIR ratio is in my mind degenerate with the (OI+CII)/FIR ratio and I would prefer not to include it in the graph.

To address this we tried to have the ratios be our measurements (i.e. CII/OI and (OI+CII)/FIR ratio), thinking that then it would only show explicity the ratios we entered as measurements. However, if we do this then attempting to run "LineRatioFit" gives us an error th_at it needs three measurements so it can make at least two ratios . . . however we've already explicitly defined the two measurements.

``` ------

A few possible solutions:

1. Add an exclude_ratio list parameter to LineRatioFit,run()
2. Add an exclude_ratio list parameter to LineRatioPlot.overlay_all_ratios()
3. Add a separate new method LineRatioPlot.overlay_ratios() that takes an explicit list of ratios.

In the meantime the kluge is to remove the unwanted ratio from the dictionaries LineRatioFit._modelratios and LineRatioFits._observedratios before plotting.