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.