Merge pull request #144 from luca-heltai/pr_tria_accessor_manifold_aware

Made TriaAccessor aware of Manifold.

This is the initial work to make TriaAccessor more aware of Manifold.

This allows (and future PR will implement this) to switch internal calls to get_manifold to syntax like

cell->center(true);
line->center(true):
line->point(p);
instead of the fairly redundant versions

cell->get_manifold().get_new_point_on_cell(cell);
line->get_manifold().get_new_point_on_line(line);

vector<Point<2> > ps(2); ps[0] = line->vertex(0); ps[1] = line->vertex(1);
vector<double> ws(2, .5);
Quadrature<2> quadrature(ws, ps);
line->get_manifold().get_new_point(quadrature);