Fixes problem for non-zero rhs and Jacobi smoothing for P2 operator

The problem resulted from an indexing issue in the function
p2::macroface::smoothJacobiEdgeDoF(). When the rhs array was accessed
it got passed indices computed with

 edgedof::stencilIndexFromHorizontalEdge()

instead of

 edgedof::macroface::indexFromHorizontalEdge()

same for the other two types of edges. This commit fixes this.