snappyHexMesh/Allrun - fit comments in 80-cols and add #!/bin/sh

tutorials/mesh/snappyHexMesh/Allrun

-# These cases are links to solver test cases and are run when the Allrun scripts of those solvers are run.
-# This empty Allrun script avoids the meshing of theses cases to be run twice.
+#!/bin/sh
+
+exit 0
+
+# These cases are links to solver test cases and are run when the Allrun
+# scripts of those solvers are run.
+
+# This dummy Allrun script avoids meshing these cases twice.
+