Added info button to seasonal chart

I've interpreted this one pretty minimally - I've added an info button to the panel header with popup text that provides an edited version of Matt's text. I've edited the text to make it slightly clearer in my opinion and provide punctuation that was missing.

I've put the title in the header of the popup. This is different to all other popups on the site, which have the word 'information' in the header and the title shoe horned into the body of the panel - I think this is not great, so have changed it here with a view to doing it to the whole site.

The issue is about providing information to assist the user in understanding it. I've interpreted Matt's suggestions as just that - suggestions, so I haven't added an additional button to the chart itself as I think it makes it too cluttered and that its much better to use the new info popup added to the header. I think we should just show it and wait for any comments.

The popup has auto positioning enabled, so by default it will appear on the right of the button, but if space is limited it will move to the left, like this: