Finish improving documentation for the Alongshore Transporter

[anardek]

I need to comment on:

• why diffusivity doesn’t matter on the -x direction
• why do we set diffusivity to zero?
• why we see an increase in diffusivity for an asymmetric wave climate (for a given shoreline orientation — for example, a straight coast — moving waves from one bin to another bin shouldn’t affect the diffusivity, because the net transport through the box is still the same. BUT, the maximum potential for diffusivity changes if the shoreline orientation changes (diagonal line). Hence the increase in the bump. Probably good to make a little figure to go along with this.