Fix a bug that gives wrong velocity near the poles.
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Merge pull request #227 from SiqiZhang/gplate_fix

Fix a bug in gplate plugin: While gplate uses zero longitude at all grid points at poles, we assume the longitude at those points vary from 0 to 360. This results in a wrong velocity to be interpolated near the poles (the velocity between poles to the first latitude grid points would be wrong, however the velocity right at the pole will be OK).
Add test for gplates-3d.
Merge pull request #228 from gassmoeller/gplate_fix

Gplate fix
fix the residuum of the iterated IMPES solver
put comment back in the right place
Merge pull request #230 from jdannberg/iterated_IMPES_fix

fix the residual of the iterated IMPES solver
Update comments in a couple of places.
Merge pull request #231 from bangerth/doc-update

Update comments in a couple of places.
add a test for the iterated IMPES solver
Merge pull request #232 from jdannberg/iterated_IMPES_test

add a test for the iterated IMPES solver
Handle change of output_interval during restart.
Merge pull request #236 from gassmoeller/visualization_fix

Always create a visualization output after restart if interval is set to 0
Increase limit for the number of inner iterations.

This is apparently necessary for some very ill-conditioned models. Specifically,
I have been running a model by Sarah Stamps with very large variations of the
viscosity, and it takes more than 2000 inner iterations to converge in this
place. Maybe not surprisingly, the run time profile looks rather unfamiliar:

+---------------------------------------------+------------+------------+
| Total wallclock time elapsed since start    |  4.37e+05s |            |
|                                             |            |            |
| Section                         | no. calls |  wall time | % of total |
+---------------------------------+-----------+------------+------------+
| Assemble Stokes system          |         2 |      82.4s |     0.019% |
| Assemble temperature system     |         1 |      44.5s |      0.01% |
| Build Stokes preconditioner     |         1 |      97.3s |     0.022% |
| Build temperature preconditioner|         1 |      2.77s |   0.00063% |
| Solve Stokes system             |         2 |  4.35e+05s |     1e+02% |
| Solve temperature system        |         1 |      6.15s |    0.0014% |
| Initialization                  |         2 |        18s |    0.0041% |
| Postprocessing                  |         1 |  1.71e+03s |      0.39% |
| Setup dof systems               |         1 |      60.4s |     0.014% |
+---------------------------------+-----------+------------+------------+

Note that we spend 99.5% of the time in the Stokes solver.
Reduce the aggregation threshold for the AMG.

On the Davies et al. benchmark (case 1.1) this reduces the time for the
Stokes solver from 436 to 352 seconds. On the case discussed previously
in this pull request, it reduces the solver time almost by a factor of 2.