Add a section discussing consequences.

This pertains in particular to the question of getting discontinuous
geometries when using manifolds and a discussion of what that means,
if it is important, and if it could be fixed.
Merge pull request #140 from bangerth/add-note-to-step-53

Discuss geometric issues.
Improve the documentation for step-52.
Merge pull request #142 from Rombur/step_52

Improve the documentation for step-52.
Added TriaAccessor::point function, which returns points on the manifold given their unit coordinates.

Simplified Manifolds::get_default_quadrature.
Fixed WB comments, and a leftover which got messed up when squashing.
Addressed last comments by WB.
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);