Example bugfix: run top hat with mpiexec

Bugfix purely in example XML: Some of the serial tests and examples have been run without mpiexec, but on some systems running with mpiexec is enforced for any program built using MPI compilers. This commit accordingly prefixes 'mpiexec' to each fluidity command line.

examples/top_hat/top_hat.xml

@@ -3,9 +3,9 @@
   <name>tophat</name>
   <owner userid="skramer"/>
   <problem_definition length="long" nprocs="1">
-    <command_line>fluidity -v2 -l top_hat_cg.flml &amp;&amp; 
-fluidity -v2 -l top_hat_dg.flml &amp;&amp;
-fluidity -v2 -l top_hat_cv.flml</command_line>
+    <command_line>mpiexec fluidity -v2 -l top_hat_cg.flml &amp;&amp; 
+mpiexec fluidity -v2 -l top_hat_dg.flml &amp;&amp;
+mpiexec fluidity -v2 -l top_hat_cv.flml</command_line>
   </problem_definition>
   <variables>
     <variable name="cg_times" language="python">import fluidity_tools