Fix sw test case

The former version had trouble reproducing the serial results in some
cases in parallel. This change replaces the input conf.gro file with
the confout.gro file from the former version of the test run in
Reference mode, because that starting configuration seems more
stable. The former input .gro file had velocities for particles named
DW, but it is not known whether this is related to the problem.

Also switched it to use ns-type=grid in line with future changes in
this area.

Refs #1429

Change-Id: Ib0e47fc2162f1e986e6d9d70d74c4aeb8fc72569