Cell size statistics should have a tooltip displaying the mass

[hhyyrylainen]

So that it is easy to see the resulting mass from the shape that ends up created for a cell (as otherwise this can be a bit hard to realize)

A bit more specific: I think also the tooltip should mention the mass depends on the final membrane shape (and isn't directly affected by the total number of hexes).

I'll mark this as easy as I think the editor already has places that need to calculate the mass so grabbing it similarly should be quite easily addable to the code as well as triggering when the organelles change like some other other placed organelle dependent stats. But it might be more accurate if this was tagged "intermediate."