Increase max dims in netcdf

This is needed for ExodusII. It is turned off by default, as it seems the
netcdf developers know about this issue:

http://www.unidata.ucar.edu/support/help/MailArchives/netcdf/msg09258.html

But they didn't change it by default.
Merge pull request #572 from hashdist/netcdf4

Increase max dims in netcdf