[PL] GWF: Explicitly set type of darcy_velocity.

auto const darcy_v = (EigenExpr).eval() returns a temporary
object. (Eigen 3.2.9).
Error is detected by clang's address sanitizer.
AddressSanitizer: heap-use-after-free on address ...

ProcessLib/GroundwaterFlow/GroundwaterFlowFEM.h

@@ -58,6 +58,7 @@ class LocalAssemblerData
 
     using NodalMatrixType = typename LocalAssemblerTraits::LocalMatrix;
     using NodalVectorType = typename LocalAssemblerTraits::LocalVector;
+    using GlobalDimVectorType = typename ShapeMatricesType::GlobalDimVectorType;
 
 public:
     /// The hydraulic_conductivity factor is directly integrated into the local
@@ -100,11 +101,12 @@ public:
                                  sm.detJ * wp.getWeight();
 
             // Darcy velocity only computed for output.
-            auto const darcy_velocity = -(k * sm.dNdx *
-                Eigen::Map<const NodalVectorType>(local_x.data(), ShapeFunction::NPOINTS)
-                ).eval();
+            GlobalDimVectorType const darcy_velocity =
+                -k * sm.dNdx * Eigen::Map<const NodalVectorType>(
+                                   local_x.data(), ShapeFunction::NPOINTS);
 
-            for (unsigned d=0; d<GlobalDim; ++d) {
+            for (unsigned d = 0; d < GlobalDim; ++d)
+            {
                 _darcy_velocities[d][ip] = darcy_velocity[d];
             }
         }