Minor documentation updates.
Also remove output files.
Give it a once over.

No functional changes, just updates to documentation and style.
Merge pull request #196 from bangerth/step-52-updates

Step 52 updates
add failing test for ghosted vectors
Allow v=0.0 for ghosted PETSc and Trilinos vectors
Allow v=0.0 for ghosted PETSc and Trilinos vectors
Merge pull request #198 from tjhei/gla_broken_ghost_vec

PETSc and Trilinos vectors with ghost entries can now be reset to zero using = 0.0