Integral on SimpleMesh not yet implemented

[mikeingold]

Ideas sought. A simple version of this might be possible by simply decomposing the mesh into a set of Triangles and then returning the sum of their integrals, i.e.:

function _integral_2d(f, mesh::SimpleMesh, rule::IntegrationRule)
    triangle = _some_function_mesh_to_triangles(mesh)
    integrand(triangle) = _integral(f, triangle, rule)
    return sum(integrand, triangles)
end