- exporters use function spaces allocated in solver class
- ExportManager has dg coordinate function to avoid allocating one in File

- Epshteyn (2007) interior penalty formulation
- used for both horizontal and vertical diffusion
- vertical diffusion is treated implicitly
- added tests against analytical solution
- updated tracer equation boundary conditions
- updated get_bnd_functions of 2d shallow water eqns

- viscosity formulation is equivalent to tracer diffusion
- using same ip penalty parameter as in 2d viscosity
- revisited test cases
- in horizontal case convergence is suboptimal, using larger slope tolerance

- limiter should preserve a linear function even at boundaries

- using top/bottom masks to compute mean value at facet triangles

- current formulation does not limit solution at lateral bnds at all