Improve Diagnostic Descriptions

[valentinaschueller]

Comment from David:
"I like the explanations at the bottom of each plot, but they are not always clear, e.g. what does 'Sum of Sea-Ice Area / siconc on northern hemisphere' mean exactly? Shouldn't it be rather something like 'Product of sea-ice concentration (siconc) and grid-cell area, summed over all grid cells of the northern hemisphere'. For sea-ice volume, it should be 'Product of sea-ice volume per area (sivolu) and grid-cell area, summed over all grid cells of the northern hemisphere'."

--> quite long descriptions! Think about where to put them in the code