Fix the barotropic streamfunction calculation

The fix involves:
* detecting boundary edges
* finding all the unique loops for each continent or island
* finding a unique list of pairs (excluding the edge that
  would close the loop
* creating a sparse matrix with:
  * A Poisson equation for the BSF in terms of the vertically
    integrated vorticity at each vertex
  * Adding a constraint on each pair of boundary vertices found
    above ensuring that their values are equal
  * Adding a constraint that the BSF at the first vertext is
    zero
  * Adding the transpose of these boundary conditions so that
    the matrix is square
* the right-hand side involves the vertically integrated vorticity
  and zeros for the boundary conditions
* the solution involves the BSF and a Lagrange multiplier (dropped)
  for each boundary condition

Author: xylar