Adds -= operator

An elementwise -= operator is added to the autodiff class.

opm/autodiff/AutoDiffBlock.hpp

@@ -194,6 +194,28 @@ namespace Opm
             return *this;
         }
 
+        /// Elementwise operator -=
+        AutoDiffBlock& operator-=(const AutoDiffBlock& rhs)
+        {
+            if (jac_.empty()) {
+                jac_ = rhs.jac_;
+            } else if (!rhs.jac_.empty()) {
+                assert (numBlocks()    == rhs.numBlocks());
+                assert (value().size() == rhs.value().size());
+
+                const int num_blocks = numBlocks();
+                for (int block = 0; block < num_blocks; ++block) {
+                    assert(jac_[block].rows() == rhs.jac_[block].rows());
+                    assert(jac_[block].cols() == rhs.jac_[block].cols());
+                    jac_[block] -= rhs.jac_[block];
+                }
+            }
+
+            val_ -= rhs.val_;
+
+            return *this;
+        }
+
         /// Elementwise operator +
         AutoDiffBlock operator+(const AutoDiffBlock& rhs) const
         {