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.