Enable use of pyclaw.io.x where x!=ascii.

This fixes a horrendous bug I introduced in https://github.com/clawpack/pyclaw/commit/08ce7b3c26f0df16ed9a700337c25ea923c4f1c4, which caused output to always be in ASCII format unless PETSc was used.

Also, fix some problems in pyclaw.io.netcdf.
I still doubt that it's working correctly, but it produces something that's ostensibly a valid netcdf file.