Make more use of move semantics in AD code.

This makes some API changes to AutoDiffBlock.
 - Add overload for the constant() constructor taking rvalue ref.
 - Add overload for the variable() constructor taking rvalue ref.
 - Make the function() constructor *require* rvalue refs.
 - Add a swap() function.

The remaining changes in this commit are follow-ups especially
to the third change (adding std::move in many places), and
some removal of unnecessary block pattern arguments from calls to
the constant() static method.

opm/autodiff/AutoDiffHelpers.hpp

@@ -403,7 +403,8 @@ collapseJacs(const AutoDiffBlock<double>& x)
     // Build final jacobian.
     std::vector<ADB::M> jacs(1);
     collapseJacs( x, jacs[ 0 ] );
-    return ADB::function(x.value(), jacs);
+    ADB::V val = x.value();
+    return ADB::function(std::move(val), std::move(jacs));
 }