Set the FE-order in finite-strain-elasticity back to: order = 1

In the default setting, we want finite-strain-elasticity to compile with dune-parmg,
and dune-parmg can only handle FE of order=1.

src/finite-strain-elasticity.cc

@@ -46,7 +46,7 @@
 // grid dimension
 const int dim = 3;
 
-const int order = 2;
+const int order = 1;
 
 using namespace Dune;