Fix GridTools::distort_random for distributed meshes
Minor documentation update. Reorder two functions.
Move communicate_locally_moved_vertices to parallel::distributed::Triangulation.
Add a test for communicate_locally_moved_vertices and verify that its output is correct.
Introduce GridTools::get_locally_owned_vertices().

Also use it in an existing test and in an assertion.
Revert to the old code for the sequential case to maintain backward compatibility. Adjust testcases.

Minor reworks.
Merge pull request #239 from bangerth/parallel-distort-random

[WIP] Parallel distort random