Combine BarotropicQG and BarotropicQGQL

[glwagner]

Continuing the discussion from FourierFlows issue #72.

At first glance, these problems have a lot in common.

In fact, I think it may ultimately make sense to include 2D turbulence in BarotropicQG, at least for the purposes of the GeophysicalFlows package. In GeophysicalFlow, 2D turbulence certainly is a type of barotropic QG flow.

Let's discuss the specific pros and cons. I'm not sure what is meant by the inclusion being 'cumbersome', but I do see some pros in avoiding the duplication of functions that calculate things like energy and enstrophy.