dynamic workslice box size for layers with low counts

[HamishB]

wishlist, semi related to #57,

for layers with just a few features on the mainland, like shoal_poly (with 8), you have to click about 50 grid boxes to select them all. It might be nice to dynamically (inverse) scale the maximum box size to the number of geographic features, since selecting all those is a bit tedious. In doing so it might help to make the code less our-dataset-centric.

thanks,
Hamish

[stephend]

#56 will also help with this.