Testsuite: Fix globbing command for all-headers tests, cleanup code

Turns out that globbing together a full path and manually extracting
relative paths is not the best idea. Clean up this mess by using
FILE(GLOB_RECURSE [base path] [...]) that produces nice relative paths.
Merge pull request #133 from tamiko/fix_header_tests

Bugfix: Fix all_header tests
Bugfix: Do not forget to use the 'RELATIVE' keyword
Merge pull request #134 from tamiko/fix_header_tests

Bugfix: Do not forget to use the 'RELATIVE' keyword
Improve assertion for access to nonlocal elements
Merge pull request #135 from kronbichler/master

Improve assertion for access to nonlocal elements
Correct wrong message from PR 135
Merge pull request #136 from kronbichler/master

TrilinosVector access assertion: Correct wrong message from PR 135
Fix two warnings with clang-3.5
Merge pull request #137 from kronbichler/master

Fix two warnings with clang-3.5
Remove a file that was probably never intended to be added.
Merge pull request #138 from bangerth/delete-outdated-file

Remove a file that was probably never intended to be added.
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.
add documentation on the effect of ConstraintMatrix::distribute_local_to_global() on eigenvalues
Improve the documentation for step-52.
Further polishing of documentation after comments
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.