Filter grid squares by area drained

Adds the functionality to filter the data by area drained by a grid square. It adds a new EdgeLayer populated with catchment data over the top of the climate data to act as a mask. Squares are made transparent to reveal the climate data beneath as the new Catchment slider is used. If a square is masked, then it has the same light blue we use on the sea. The catchment data has been log transformed to help give a smooth 'reveal' when the slider is used, otherwise the reveal becomes extremely bunched to the lower end of the slider, with a long tail catering for the small number of very large draining squares.

Here's what it looks like with the slider at an intermediate value to reveal major rivers:

And here's the tool tip: