do not try to read aux files.

Default in pyclaw/solution.py apparently change with read_aux=True being the default, causing error messages to print from visclaw for problems with aux arrays.

Avoids output like this...

```
Now making png files for all figures...
2014-06-11 16:49:21,204 INFO CLAW: Unable to open auxillary file /Users/rjl/git/clawpack/amrclaw/examples/advection_2d_annulus/_output/fort.a0000 or /Users/rjl/git/clawpack/amrclaw/examples/advection_2d_annulus/_output/fort.a0000
    Reading  Frame 0 at t = 0  from outdir = /Users/rjl/git/clawpack/amrclaw/examples/advection_2d_annulus/_output
Frame 0 at time t = 0.0
```